Let’s kick off the review session by addressing a confusion that will get you relentlessly and uninterestingly mocked: homophonic pairs. These are pairs like your and you’re or affect and effect, which are pronounced the same but spelled differently. Even if you know the difference between them, you’re still going to screw them up occasionally, especially in quick emails or when you’re writing with your attention wandering. (I probably type the wrong one about 1% of the time, which doesn’t sound like too much until you think about how often one of these words gets used.)
I’m going to look at a subset of these homophonic pairs here, the ones where one member of the pair is a contraction. These are the aforementioned your/you’re, as well as their/there/they’re, its/it’s, and whose/who’s.
In all four of these cases, the word with the apostrophe is the one that can be written as two words. You’re is the contraction for you are, they’re is they are, it’s is it is, and who’s is who is. Thus:
(1a) Do you mind if I dance with your date?
(1b) It seems you’re [you are] offended for some reason.
(2a) I think those tourists left their suitcases behind.
(2b) Yup, those suitcases over there.
(2c) Let’s see if they’re [they are] full of money.
(3a) I couldn’t open the suitcase because its lock was too strong.
(3b) I know, it’s [it is] a shame.
(4a) Do you know anyone whose skill set includes lock-picking?
(4b) Wait, who’s [who is] a cop?
Pretty straightforward, right? The words with the apostrophes are always contractions of two words, a pronoun and a form of the verb be. The words without apostrophes are possessives (and also the locative there). It seems like you ought to just remember apostrophes = two words, no apostrophes = possessives. Easy peasy.
But if it’s so easy, why is it so hard? The trouble is that these homophones don’t exist in a vacuum, and the rest of English exists to sow confusion. When you think of forming a possessive, no doubt your first thought is of the apostrophe-s. That’s because most (singular) nouns are made possessive with apostrophe-s: rabbit’s foot, someone else’s fault, etc. As a result, it’s and who’s look possessive even though they’re not.
The trick is that (personal) pronouns never use apostrophe-s in their possessive forms; in fact, many of them don’t even use an s.* They have their own special forms: my, your, his, her, its, our, their. If you remember that pronouns don’t take apostrophe-s, then its/it’s and whose/who’s are a lot easier to decide between.
Another way to think about it is that only one member of the pair can have the apostrophe (otherwise there’d be no confusion). And connecting two contracted words needs an apostrophe more than signalling possession does. Since there’s only one apostrophe to go around, the contraction gets it over the possessive.**
Summary: If your and you’re or it’s and its are confusing you, remember that contractions always have an apostrophe, and possessive pronouns never do.
*: Never say never. Impersonal (one) and indefinite (e.g., everybody) pronouns do take apostrophe-s. Luckily, these ones don’t have as prominent of homophones and don’t cause many problems for writers.
**: Of course, that’s merely a mnemonic. There is no rule of English that says this, and the historical development that led to pronomial possessives not having apostrophes was not a result of this.