Case is the grammatical function of a noun or pronoun. There are only three cases in modern English, they are subjective (he), objective (him) and possessive (his). They may seem more familiar in their old English form - nominative, accusative and genitive. There is no dative case in modern English. Yippee!

First more good news. You cannot really go wrong here, we got rid of most of our cases and as a result English is easier than many other languages because nouns and some indefinite pronouns (anyone, someone, everyone, and so on) only have a distinctive case form for the possessive. There are a few remnants of old English though, and pronouns have distinctive forms in all three cases and should be used with a bit more care.

The pronoun cases are simple though. There are only three:-

1. Subjective case: pronouns used as subject.
2. Objective case: pronouns used as objects of verbs or prepositions.
3. Possessive case: pronouns which express ownership.

Personal Pronoun

Subjective/Nominative Objective/Accusative Possessive/Genitive
Referring to the subject in a sentence Referring to the object in a sentence The apostrophe form of the word ("Lynne's).
I Me Mine
You You Yours
He Him His
She Her Hers
It It Its
We Us Ours
They Them Theirs
Who Whom Whose

These pronouns, and who and its compounds, are the only words that are inflected in all three cases (subjective, objective, possessive). In nouns the first two cases (subjective and objective) are indistinguishable, and are called the common case. One result of this simplicity is that, the sense of case being almost lost, the few mistakes that can be made are made often, even by native speakers, some of them so often that they are now almost right by prescription.