The apostrophe is used in English to indicate possession, as well as being used for contractions—going in place of letters when a word or words are shortened for speaking purposes—fuckin’ zombie holocaust instead of fucking zombie holocaust. For possession, you would use an apostrophe when writing the zombie’s student loan, to indicate that the student loan belongs to the zombie. If there are TWO zombies, you say “There are two zombies.” Note the absence of an apostrophe. It’s only when these (plural) zombies own things that they get an apostrophe after the s: the zombies’ student loan(s). However, there is an exception to this rule—if the plural of the word that is owning the thing being possessed (fuck this is hard to explain—Imean the word you have to put the apostrophe on) is formed without an s, then the apostrophe goes before the s:
One zombie’s student loan. Two zombies’ student loans. One person’s brain being eaten. Two people’s brains being eaten.
So basically, if you can rearrange the sentence to use of instead of an apostrophe (the man’s zombie-dog—the zombie-dog of the man) and there isn’t an s on the end, then you need an apostrophe and then an s. If there is an s on the end (the students’ rotting flesh—the rotting flesh of the students), then you just put the apostrophe after the s that is already there (Also—James’ dismembered foot). This is all pretty straightforward.
It all gets confusing with the unfortunate contraction it’s, which ALWAYS means it is. This is commonly confused with the possessive adjective its, because of the apostrophe which is usually used for possession. But its is possessive in itself and does not require an apostrophe—just like the other possessive adjectives my, your, his, her, our and their.
So how do you know if you should use it’s or its? If you can change the its/it’s in question to it is and your sentence makes perfect sense (sorry, I should say, that bit of your sentence makes sense), then you need the apostrophe. Alternatively, if you can make whatever it is masculine or feminine and replace the its/it’s with the possessive adjective his or her, then you don’t need the apostrophe:
There’s the reanimated human corpse; that’s its cellphone.
There’s the reanimated human corpse; that’s his/her cellphone.
Questions? Send me an email at mikey@salient.org.nz.
Braaaaaaaaaiiins.
Suggested reading:
Books in general (about zombies).
Wikipedia.
Some sort of book on grammar. Find one yourself.