This post was inspired by Robin Koerner’s little rant in the Huffington Post about his encounter with a job applicant who had a 3.9 GPA but no idea of how to use an apostrophe correctly.

While you can, in real life, break some of the grammatical rules the SAT and ACT test without anyone really noticing, you cannot break this one. Sure, your high school or even your college teachers may overlook it, but if you screw this up on your resume or even in an email to a potential employer or college interviewer, it may very well be noticed and count very seriously against you. In many cases, it can lead to a flat-out rejection. Employers actually don’t care about your critical-thinking skills unless you can express yourself in basic, coherent English. If you don’t believe me, check out this article from the Washington Post.

Of all the basic rules to mess up, apostrophe usage will stick out the most, so if you learn even one thing from SAT/ACT prep, please let it be this. Besides, apostrophes are always tested; mastering them can get you an easy couple of points.

An -s by itself is used to make a noun plural. No apostrophe is needed.

Incorrect: I have two dog’s at home.
Correct: I have two dogs at home.

An apostrophe is needed, however, to make a noun possessive — that is, to indicate that it belongs to someone or something.

Incorrect: This is my dogs toy.
This is my dog’s toy. (= This is the toy that belongs to my dog.)

An apostrophe is also needed when forming a contraction between a noun and the verb is.

Incorrect: My dogs almost as large as I am. 
My dog’s almost as large as I am. (= My dog is almost as large as I am).

When a noun is both plural and possessive, the apostrophe is placed after the -s.

Incorrect: These are my dogs toys.
Incorrect: These are my dog’s toy’s.
Correct: These are my dogs’ toys. (= These are the toys that belong to my dogs.)

Irregular nouns do not take an -s in the plural form.

Child, Children
Foot, Feet
Fish, Fish

To form the possessive of an irregular plural noun, add apostrophe + -s.

Incorrect: The childrens game
Incorrect: The childrens’ game
Correct: The children’s game

When you encounter these types of questions on the ACT or SAT and are not immediately sure of the answer, you need to break them down into steps.

First you need to determine whether you are talking about something possessive. That will determine whether you need an apostrophe at all.

Next determine whether the noun in question is singular or plural. If it’s singular, the apostrophe comes before the -s. If it’s plural, the apostrophe comes after.

You can also use this shortcut: a possessive noun (apostrophe) must be followed by another noun, so if you’re not sure whether an apostrophe is necessary, looking at the following word. If it’s a noun, you need an apostrophe; if it isn’t, you don’t.

So, for example, the apostrophe in the phrase “book’s cover” is correct because cover is a noun, but the apostrophe in “book’s are” is incorrect because are is a verb.

Let’s consider the following (real) ACT sentence:

The sound of the distant honking of these majestic birds always makes me look up.

G. birds,
H. bird’s
J. birds’

The first thing we need to figure out is whether birds should in fact be plural, and the word these indicates that it should. (You wouldn’t say These majestic bird). So we definitely need an -s on the end of birds. The only question is whether there needs to be an apostrophe.

To determine whether the apostrophe is necessary, look at the word after birds. A possessive noun must be followed by another noun, and always is not a noun. As a result, no apostrophe is necessary. That eliminates H and J. Because no comma is necessary, the answer is F.