I don’t know what they told you, but when I was in school, they told me that the apostrophe is used to make the possessive in English.
That’s a crock. The apostrophe is involved in the possessive in English ONLY incidentally.
In fact, as far as I’ve been able to determine so far, the apostrophe means one thing, and one thing only:
That is, the apostrophe is only for contractions.
Our/Ours, Your/Yours, Its, Mine, Her/Hers, His, Their/Theirs, Whose: these are all, quite clearly, possessives. Nary a one of ’em needs or gets an apostrophe.
In connection with this, I have a pet peeve or two and a word of explanation. Let’s do the explanation first.
The possessive, aside from things like the ones I’ve already mentioned, is a very ancient construction that goes about like this:
(This is a matter of public record, you know. You don’t have to believe me, go check it for yourself. If you read people like Ben Jonson, you will find constructions on the order of “...Matho his soldiours....” -- Jonson didn’t even use a comma in that one.)
As to “it’s”, that and the others like it (who’s, where’s, that’s, etc.) are contractions for xxx is and xxx has. They, too, have nothing to do with the possessive.
A peeve: I see things like “back in the 80’s” fairly frequently. Stop and think for a moment: what are you leaving out? In this construction, what is being left out (if anything) is the CENTURY. The apostrophe goes BEFORE the number, not after it! (If you are saying something on the order of "A man in his 80s," you don't need an apostrophe at all, unless you are writing science fiction. In the early 21st century, we don't generally get to see a second set of eighties.)
A peeve: I also see plurals with apostrophes in them: I saw “Entree’s” today, for example. Stop and think. Am I leaving anything out? If not, what is the apostrophe doing in there? You don’t just sprinkle them in at random, y’know! They’ve got a job to do.
If you think you have a legitimate use for the apostrophe that fails
to meet my rule, please let me know. You can reach me via email: jon
[at] bazilians [dot] org