14 Essential Grammar Rules for High School Students (Without the Fluff)

Bonus: $50 Discount on PrepMaven’s Essential Grammar Workshop

Proper grammar is the foundation of strong writing. 

Yet many high school English classes don’t include grammar components. If they do, they are either too brief or too technical to be effective in the long run.

Given that high schoolers have to apply knowledge of grammar rules to the SAT and ACT and need proficiency in certain rules to succeed in college, we’ve done two things:

  1. Written this post about the most essential grammar rules high school students need for success
  2. Compiled an entire summer workshop series devoted to essential grammar rules and style

You can get $50 off one-time enrollment in this workshop right now. Otherwise, keep on reading to dive further into these essential rules.

Here’s what we cover:

  1. 14 Essential Grammar Rules
  2. PrepMaven’s Essential Grammar Summer Workshop
    • Bonus: $50 Discount for One-Time Enrollment

14 Grammar Rules You Need to Succeed

We’ve spent years working with high school students in academic writing, SAT/ACT prep, and college essay writing. Time and again, we’ve seen the same 14 grammar rules come into play in all of these areas.

We also know that grammar itself can be technical and boring. That’s why we outline the rules in this post in a simple and straightforward way, without jargon or “fluff.”

Rule #1: A complete sentence has a subject, a verb, and full expression of a thought.

Understanding the difference between complete and incomplete sentences is crucial for so many things. 

It’s especially valuable for applying most of the punctuation rules discussed in this post. 

A complete sentence must have the following three things:

  1. Subject
  2. Verb
  3. The complete expression of an idea

A subject is a person, place, thing, or idea. Examples of subjects include apple, optometrist, dyslexia, and the United Kingdom. A verb expresses the action of a subject, such as is, completed, running, and have.

Now, what do we mean by “the complete expression of an idea”? Basically, this stipulates that the sentence doesn’t leave you hanging. It expresses a full idea.

Here’s an example of a complete sentence that expresses a full idea:

Cherise decided to travel to the United Kingdom and seek employment after she completed her teaching certification.

The subject of this sentence is “Cherise,” while the verb is “decided.” The sentence fully expresses the idea that Cherise made a choice to travel to another country following completion of her teaching certification.

Yet a sentence doesn’t have to be super long to express a full idea. Check out these sentences that are, in fact, complete:

I understand.

She couldn’t go.

David waited.

In grammar language, a complete sentence is called an independent clause. Clauses contain a subject and a verb. 

If a sentence doesn’t have one or more of the three things needed for a complete idea, it’s incomplete. If it contains a subject and a verb, but not a full expression of an idea, it is called a dependent clause.

Here’s an example of a trademark dependent clause:

Although I intended to sign up for PrepMaven’s Essential Grammar Workshop series

Notice how this sentence still has a subject (“I”) and a verb (“intended”), which makes it a clause. However, the sentence does not express a full idea. In fact, it leaves us hanging! We know this person intended to sign up for the summer workshop, but the rest of the story is missing.

That word “although” is the secret culprit behind the incompleteness of this sentence. Subordinate conjunctions like although always make a sentence incomplete!

Here’s a list of common subordinate conjunctions. When you see these words at the start of a sentence, be on the lookout for an incomplete idea:

while after because although before
unless as if since when
whenever whereas even though rather than until

Here are additional examples of incomplete sentences:

The long-awaited decision to appeal

While the rest of the class worked on the exam

Horses running through the field on a cloudy day


Rule #2: Combine two complete sentences with either a period, a semicolon, or a comma + FANBOYS conjunction.

That’s right! If you are trying to join two complete sentences to create what is called a compound sentence, you can only do so with one of the following punctuation options:

  1. Semicolon
  2. Period
  3. Comma + FANBOYS conjunction (for, and, nor, but, or, yet, so)

Here is the same compound sentence written three ways to prove this point:

It’s not that people are disinterested in climate change. Many would argue that citizens are very interested in the planet’s gradual warming.

It’s not that people are disinterested in climate change, for many would argue that citizens are very interested in the planet’s gradual warming.

It’s not that people are disinterested in climate change; many would argue that citizens are very interested in the planet’s gradual warming.

Two Tips About Semicolons

Semicolons only like to hang out between 2 complete sentences. Yet, occasionally, they can come before a transition word like “however,” “nonetheless,” or “moreover.”

This is perfectly acceptable, as long as that transition word has a comma after it, as in this example:

It’s not that people are disinterested in climate change; indeed, many would argue that citizens are very interested in the planet’s gradual warming.

Additionally, FANBOYS conjunctions (for, and, nor, but, or, yet, so) never follow a semicolon.


Rule #3: Use a comma to join a dependent clause to an independent clause (most of the time).

To create what is called a complex sentence, use a single comma. This means linking together a dependent clause with an independent clause.

Here is an example of a complex sentence that includes a comma:

In light of the fact that women are still earning less than men in the workplace, equity consulting companies are likely to prove their value in years to come.

There are cases where a comma is not needed, but this depends on the dependent clause, as in this example here:

I won’t go to the store until I have finished my work.

“I won’t go to the store” is an independent clause; “until I have finished my work” is a dependent clause. However, no comma is necessary here.


Rule #4: A colon must come after a complete sentence.

The sentence that precedes a colon must be complete. A colon also introduces a list, explanation, definition, and/or elaboration, as in this example here:

Based on these facts, some might conclude that Shakespeare was, in fact, the opposite of who he was allegedly acclaimed to be: not an original writer but, rather, a clever plaigarist.

Additional Tip: A single long dash functions much like a colon in that it must also come after a complete sentence. It differs from a colon, however, in that it must precede a new thought, interruption, or clarification.

Rule #5: Use 2 commas to separate nonessential information from the rest of the sentence.

Just as we use parentheses to separate additional information from the rest of a sentence, we can use 2 commas to accomplish the same goal.

What is “additional” or “non-essential” information?

This includes anything that is not essential for making a sentence complete (i.e., a subject, verb, or words that contribute to the full expression of an idea), such as descriptive phrases and transition words. Essentially, if you get rid of this information, you’ll still have a complete sentence.

Take a look at this example:

Greg and Kevin, the co-founders of PrepMaven, emphasize the importance of character when it comes to college applications.

Can you spot the non-essential information in this sentence? If you guessed “the co-founders of PrepMaven,” you’re absolutely right!

“The co-founders of PrepMaven” is a descriptive phrase that provides more information about the sentence’s subject, “Greg and Kevin.” It is not needed to make the sentence complete. In fact, we could get rid of it and still have a complete sentence!

Greg and Kevin emphasize the importance of character when it comes to college applications.

Additional Tip: You can use two long dashes in exactly the same way to separate nonessential information from the rest of the sentence.

Rule #6: Use commas to separate items in a list.

When listing out items, use a comma to separate each item. If you follow British English (and some academic writing styles), leave out the comma before the “and.”

However, if prepping for the SAT/ACT or following American English, incorporate the comma before the “and,” as in this example:

Before I leave for the holidays, I need to find a babysitter, submit that annual report, and RSVP for the Christmas party.


Rule #7: Place a comma after a transition word or introductory phrase.

A “phrase” is simply a group of words. Phrases are different from clauses, which contain a subject-verb pair (and can be complete or incomplete). An introductory phrase begins a sentence, often providing context, time or location cues, or transitions, as in the following examples:

In 1938

On the other hand

Beneath the sofa 

If you start a sentence with an introductory phrase or transition word, you need to place a comma after it. That’s all there is to it!

In 1938, historians were only just starting to comprehend the impact of the changing times.

On the other hand, such precautionary measures are ones we should be taking on a daily basis, not merely in times of crisis.

Beneath the sofa, Lucy found a panoply of forgotten items, including a keyring, dog toy, and grocery receipt.


DID YOU KNOW? We cover all of these essential grammar rules and more in our summer workshop. For a limited time, we’re offering a $50 discount for one-time enrollment, which you can grab below.


Rule #8: Use apostrophes to show possession with plural and/or singular nouns (and contractions).

We use apostrophes to show possession and contraction. When it comes to possession rules, keep the following in mind:

  • Add an ‘s to singular nouns showing ownership
  • Add a single apostrophe to plural nouns showing ownership

Check out these examples of singular noun possession:

  • Dmitri’s dreams
  • The cat’s favorite window sill
  • The Earth’s curvature

How can you tell if a noun is singular? There should be only one of that particular noun. For example, there is only one Dmitri, one cat, and one Earth in the sample phrases above.

Here are some examples of plural noun possession:

  • The books’ covers
  • The sidewalks’ cracks
  • My teachers’ curriculum

Besides the fact that these plural nouns end in “s,” you can tell that they are plural because there is more than one of each. From the examples, we know that we are discussing more than one book, sidewalk, and teacher.

What about singular nouns that end in “s,” including proper nouns like Chris? You still follow the rule of adding an ‘s to these nouns. Here’s what that would look like:

  • Chris’s classes
  • The iris’s stamens
  • The sea bass’s flavor

We know it feels awkward, but that’s the rule! The only exception to this is with proper nouns that have historical and/or biblical associations, like “Moses” or “Jesus.” In these instances, all you need to do is add an apostrophe to the end:

  • Moses’ leadership
  • Jesus’ teachings

You can have a plural noun that doesn’t end in “s”. What happens if you want to show possession with one of these nouns? All you need to do is treat it like a singular noun: add an ‘s to the end. Check out these examples:

  • The children’s games
  • People’s voting habits
  • Women’s rights

Rule #9: Verbs must match their subjects (and vice versa).

This is the heart of subject-verb agreement: verbs must match their subjects!

But what do we mean by “match”? Verbs must match their subjects in form, which is different from tense.

Here’s what that generally breaks down to:

  • A plural noun must have a plural verb
  • A singular noun must have a singular verb

A plural noun is a noun that indicates more than one of some thing, idea, or individual: horses, children, mosses. A singular noun indicates that there is only one of some thing, idea, or individual: horse, child, moss.

Now, even though we don’t always think of verbs in terms of their singularity or plurality, a verb will change form depending on whether its noun is plural or singular. Take a look at the following examples to see this in action:

  • The horses run across the field.
  • The horse runs across the field.
  • These mosses are hard to identify.
  • This moss is hard to identify.

In the first two examples, the plural noun (horses) matches the plural verb (run), while the singular noun (horse) matches the singular verb (runs). Run and runs are different verb forms.

In the second two examples, the plural noun (mosses) matches the plural verb (are), while the singular noun (moss) matches the singular verb (is). Are and is are different verb forms.


Rule #10:  A pronoun is a word that takes the place of a noun. 

We use pronouns so that we don’t have to say the same noun over and over again in a sentence or paragraph. That’s what makes them so useful!

There are several different types of pronouns. We’ve outlined the most common types in the chart below.

Subject Pronoun Object Pronoun Possessive Adjective Possessive Pronoun Reflexive Pronoun
I me my mine myself
you you your yours yourself / yourselves
we us our ours ourselves
they them their theirs themselves
she her her hers herself
he him his his himself
it it its n/a itself

A pronoun must match its noun in both type and form. For example, an object pronoun (me, you, us, them, her, him, it) must replace a noun that functions as a direct object. The same goes for subject pronouns, possessive adjectives, possessive pronouns, and reflexive pronouns.

Here’s a list of pronouns and their nouns (called antecedents) that demonstrate this correlation:

  • people’s voices –> their voices
  • Give the gift to Roger  –> Give the gift to him
  • I don’t know anything about trigonometry –> I don’t know anything about it
  • Ms. Lutz is teaching the class –> she is teaching the class
  • This book is Susan’s –> This book is yours

All of the pronouns in these examples match their nouns (antecedents) in type and form. We wouldn’t replace, for example, “people’s” with “hers” or “trigonometry” with “them.”

One of the most common pronoun mistakes confuses object pronouns for subject pronouns, as in the following incorrect sentences:

  • Her and I plan on traveling to Uruguay soon.
  • Mr. Banks, whom is teaching the class, has a wide range of advanced degrees.

In these sentences, the writer is erroneously using an object pronoun in place of a subject pronoun. If you ever are unsure about the difference, simply replace the pronoun with a noun to test it out:

  • Kate and I plan on traveling to Uruguay soon –> Kate = “she” –> She and I plan on traveling to Uruguay soon.
  • Mr. Banks is teaching the class. –> Mr. Banks = “who” –> Mr. Banks, who is teaching the class, has a wide range of advanced degrees.

Rule #11: Modifiers must be placed correctly.

A “modifier” refers to a word or group of words that provide more information about a certain subject. In most Modifier questions on the SAT or ACT, a modifier is a descriptive phrase that provides additional details about a subject, often appearing at the start of a sentence.

Here are some examples of modifying phrases that might offer further details about a specific subject:

  • walking down the street
  • bespectacled and grimacing
  • associated with ancient tradition

In general, modifiers and modifying phrases must be right next to whatever it is they are modifying! Take a look at these sample sentences that use the modifying phrases above:

  • Walking down the street, my friend and I took in the gorgeous sunset.
  • Bespectacled and grimacing, the professor made his way to the lectern.
  • Associated with ancient tradition, the practice of ancestral worship appears in many cultures.

In all of these examples, the modifying phrases are placed directly next to the subject they are modifying. Misplaced modifiers are called “dangling modifiers,” and these incorrectly modify a subject. Here are the same examples written with dangling modifiers:

  • My friend and I took in the gorgeous sunset, walking down the street. 
  • The professor made his way to the lectern, bespectacled and grimacing.
  • The practice of ancestral worship appears in many cultures, associated with ancient tradition.

Technically, due to the misplacement of modifiers, these sentences declare that it is the sunset that is “walking down the street,” the lectern that is “bespectacled and grimacing,” and the cultures that are “associated with ancient tradition.” In reality, the friends are walking down the street, the professor is bespectacled and grimacing, and the practice of ancestral worship is associated with ancient tradition.


Rule #12: Apply parallel structure for comparisons or items in a list.

Parallelism isn’t just a concept that appears in math. It also concerns grammatical structure, especially with respect to comparisons or lists.

To apply parallel structure, ensure that items in a comparison or list are in the same form, category, and/or number.

In this next example, the verbs in the list all have the same form (-ing):

Desmond spends his days at the library photocopying, transcribing, and cataloguing articles.

Here, the nouns in the list are all singular, reflecting parallelism in number:

Someday I hope to invest in a car, a retirement account, and a rental property.

Lastly, in this comparison, notice how the sentence compares nouns that are the same “category:” i.e., the “car enthusiasts of this show” and “those” of “past events.” This is parallel structure, too, as it’s technically grammatically incorrect to compare things that do not follow the same category.

The car enthusiasts at this road show, however, seemed far less interested than those of past events.


Rule #13: Who vs. Whom

A lot of students get hung up on the difference between these two pronouns. Yet thinking about them as the pronouns they are can be helpful for telling them apart.

“Who” is a subject pronoun, while “whom” is an object pronoun. 

This means that “who” can only ever take the place of a noun that acts as the subject of the sentence. “Whom” can only replace the noun that functions as a direct object in the sentence.

  • I made the painting for Cherise, who is in charge of funds allocation.
  • After the lecture, the professor spoke to the student with whom he is conducting collaborative research.

To test to see if you are using the appropriate pronoun, replace “who” with another subject pronoun like “she” or “they;” replace “whom” with an object pronoun like “him” or “them.” This will usually reveal the right choice.


Rule #14: Lay vs. Lie

It can be similarly challenging to distinguish between these two verbs. We encourage students to think about them by their definitions:

  • “Lay:” to place something (or someone) down
  • “Lie:” to actually be in a prone position

If you use “lay” in a sentence, this verb has to be stuck to a direct object, as in this example:

I lay my pens, papers, and note-taking materials on the table.

If you use “lie” in a sentence, the verb does not need a direct object, as in this example:

I think I’ll lie down right here on this patch of grass.

PrepMaven’s Essential Grammar Summer Workshop Series

Essential Grammar Rules Workshop

We’ve built the curriculum of our Essential Grammar Summer Workshop series around these 14 essential rules. In this course, students will

  • Boost their proficiency in these essential grammar rules
  • Understand these rules’ relationship to style
  • Improve their overall English mechanics
  • Incorporate what they’ve learned into their academic writing

Learn more about the workshop here.

You can also get $50 off a one-time enrollment fee here.