Objective pronouns are used as the objects in sentences. You would say, for instance:
Jennifer came to see her last night.
For the twins' birthday, Amy gave them several new toys.
As with compound subjects, problems arise when there are compound objects. People sometimes write or say sentences like this:
The argument arose last night between Carla and she.
Please buy a raffle ticket from Nancy or I.
Each pronoun is used incorrectly in these sentences. The pronouns used as subjects here should all be in the objective case: me, you, him, her, it, us and them. So, the sentences should read:
The argument arose last night between Carla and her.
Please buy a raffle ticket from Nancy or me.
You can use the same trick that you used for the subjective pronoun problem, but substitute the objective form; that is, write or say the sentence with only one object. You'd never say:
The argument arose last night between she.
Please buy a raffle ticket from I.
Since those pronouns sound wrong when they're by themselves, you know that they're the wrong case. Change the pronouns to the ones you'd normally say when there is only one object.
So why were you and it on the lists of both subjective and objective pronouns? Because, unlike other pronouns on the lists (I and me, for example), English uses the same form for those two words.
It was nice to get a surprise in the mail. (It used as a subject.)
I got it in the mail. (It is used as an object.)
You called me a four o'clock? (You is used as a subject.)
I called you back at five o'clock. (You is used as an object.)
Source: The Only Grammar Book You'll Ever Need