I covered who vs. whom in Grammar Rodeo #1. I also spoke briefly about possessive its in Grammar Rodeo #2. Drew is correct; the only time you use an apostrophe is when it's is short for it is. So if you read your sentence and it is can be substituted for for the its/it's, then you should be using the version with the apostrophe. If it is being given possession of something, you just add s. That makes NO sense, as every other time you give a someone or something a possession when writing in English, you use the apostrophe s... but no one has EVER said English makes a lot of sense.

When I researched it before, there was one view that it's as a contraction, it is, was the more common usage and so the apostrophe s was excluded from its for possession through history until that became the common usage. The other was that the word "it" is a pronoun and the apostrophe s isn't used for pronouns like hers, his, theirs, so not using the apostrophe s when its is being used as a pronoun is consistent with that practice. Of course, when one is used a pronoun and is possessive the use is one's... so like I said, I think mostly it's just that English is a language geared to have no consistency because someone, somewhere in time, was a total sadist.

One compound subjects, I've had editors disagree on how to manage possessives.

A. We drove to Matt and Paul's house, or

B. We drove to Matt's and Paul's house.

I like the questionaire in the post, because the first example shows the complexity. I believe 1A is correct because it's "use" that's the subject, which is singular -- the types of objects being used is descriptive and for clarification and are not the subject. Am I right?

Graeme, you're correct. In that sentence, the "use" of the cellphones and mp3 players is the subject, so you'd use the singular version of "is".

The use of apostrophe s with compound possessives is a bit tricky, and in the end BOTH are right--depending on what/who is doing the possessing and what they're possessing. The rule is that if the nouns are closely linked that are doing the possessing, then you only use apostrophe s on the 2nd noun. If the nouns are not closely linked, you would use apostrophe s on both.

Your example, "We drove to Matt and Paul's house" would use the possessive apostrophe s only after the 2nd name because it is a singular thing they both own "together".

You can have a singular name for objects that are still owned separately like:

"I've used New York's and Baltimore's public transportation system."

Those are both distinctly different things (even if you call them the same name) being possessed by two separate cities, so you'd use the apostrophe s for both cities.

This is per Chicago Manual of Style 7.72, Joint vs. Separate Possession

Another good grammar fail!   The funny thing is, "you" can be singular or plural as a pronoun... but when used as the verb form it's always considered plural, and so "are" is correct instead of "is" even if the sentence only refers to one person.

You as a group:

"You are going to line up quietly," the teacher instructed her class.

You as a single person:

"You are going into the store in those?" she asked.

I grinned. "Sure, so what if my pajama pants have ducks on them?"

Well if you are prostituted, maybe you would have the money so you wouldn't need to shoplift

Just saying