Once you have a firm grasp of the parts of a sentence, non-essential phrases, and the eight comma rules, you should be able to confidently determine if a comma is necessary or not.

That said, it can be helpful to point out common comma mistakes so you can easily recognize them on the ACT (and in your own writing). After each mistake, we will explain what makes it erroneous.

1. Comma Splices

You have probably heard a teacher mention "comma splices" before. A comma splice is a comma that separates two independent clauses without the use of a coordinating conjunction (FANBOYS).

Here are a few example sentences with comma splices:

I took my dog for a walk, my brother mowed the lawn.

Kei kept looking over at her classmates' exams, the teacher accused her of cheating.

There are comma splices in both of the above example sentences.

To fix the error, we could add coordinating conjunctions to both sentences:

I took my dog for a walk, and my brother mowed the lawn.

Kei kept looking over at her classmates' exams, so the teacher accused her of cheating.

2. Separating essential information from the sentence

We learned in Comma Rules that we can separate non-essential information using commas.

The new park, of course, is a popular tourist destination.

However, if the meaning of the sentence were to change when a phrase is removed, then it is essential information. We do not use commas to offset essential information.

Example:

My little brother loves to play Monopoly with his friends.

not: My little brother loves to, play Monopoly, with his friends.

“play Monopoly” can’t be removed from this sentence because “My little brother loves to with his friends.” is not a complete sentence on its own.

Example:

When pro golfer John Smith won the U.S. Open, the crowd cheered.

not: When pro golfer, John Smith, won the U.S. Open, the crowd cheered.

“John Smith” can’t be removed from this sentence because “When pro golfer won the U.S. Open, the crowd cheered.” is not a complete sentence on its own.

3. Separating words/phrases that modify each other

You may encounter questions where a comma is separating a subject from its verb, an adjective from its noun, etc. These questions may also introduce this error in one of the seemingly correct answer choices. Because these modifiers are essential to the meaning of the sentence, you should not use a comma.

Example:

The police officer walked down to the corner.
not: The police officer, walked down to the corner.

We would not separate the subject ("police officer") from the verb ("walked") because it's essential information.

Example:

The blue-shirted girl sat in front of me on the bus.
not: The blue-shirted, girl sat in front of me on the bus.

The adjective "blue-shirted" describes the noun "girl," so we do not need a comma. However, in this sentence:

The blue-shirted, Snickers-eating girl sat in front of me on the bus.

We would use a comma between "blue-shirted" and "Snickers-eating" since they are both adjectives of equal weight describing the girl.

Example:

The article I read last night reported that “the youth are rising up in protest.”

Preceding the quotation with the word “that” integrates the quotation, now a subordinate clause, into the sentence completely, and it is incorrect to insert a comma between the word “that” and the rest of its subordinate clause. By using the word "that," you have now made "that 'the youth are rising up in protest.'" into essential information.

4. Using commas + FANBOYS when there is a dependent clause

As we learned in Comma Rules, we can separate two independent clauses using a comma + coordinating conjunction (FANBOYS). A mistake that is often overlooked is the sentence that inserts a comma when the first or second clause is dependent (lacking a subject, verb, or object).

Example:

Jeff likes to relax on his couch andlisten to music.
not: Jeff likes to relax on his couch, and listen to music.

"Jeff likes to relax on his couch" is an independent clause. "Listen to music" is a dependent clause because it lacks a subject. When you have an independent clause and then a dependent clause, you do not need a comma.

Here are two helpful templates to remember:

IC, FANBOYS IC
IC DC

5. Commas with tricky quotations

If a quote is a complete sentence or explanatory, we do not need to use a comma. We can either use a period (if the sentence is not an explanation or extension of the previous sentence) or a comma (if the sentence is an explanation or extension of the previous sentence).

Here's an example of when to use a period:

I didn't know what to do. "Let's go to the beach house," Marnie said, "and we can figure it out from there."

We used a period after "I didn't know what to do" for two reasons. First, "Let's go to the beach house" is something that Marnie said, not what the first person "I" was saying or thinking. Second, because "Let's go to the beach house" is not an extension of our first sentence, we used a period.

Here's an example of when to use a comma:

Glancing down at the instructions, I found the following advice on how to assemble my new bookshelf: "First, gather the necessary tools. Next, take the…”

We used a colon here because "First, gather the..." sentence is both an independent sentence and an extension of the previous thought ("...following advice on how to assemble my new bookshelf").