Pronouns must be in the proper case.
Even if this is the first time you've ever seen this term, you're probably already familiar with the concept. You would never say, "me went to the store," for example. Questions of case include who vs. whom, us vs. we, myself vs. me, etc.
The personal pronouns (enumerated below), as well as who and whom, change form as they function as the subject or object of a sentence. Cases for these pronouns are as follows:
(refers to subject):
|you||you||your, yours||yourself, yourselves|
Thus, pronoun subjects, including the subjects of noun clauses, must be in the nominative case. For example:
1. I throw the ball.
I is the subject of the verb throw.
2. Do you suppose we will win the game?
We is the subject of the verb will win.
3. Give it to the person who you think deserves it.
Who is the subject of the verb deserves; you think is parenthetical.
Likewise, pronoun objects of verbs and prepositions are in the objective case. For example:
1. They throw the ball to me.
Me is the object of the verb throw...to.
2. Twenty of us went to watch the game.
Us is the object of the preposition of.
3. To whom did you give it?
Formal English requires whom, which is the object of the preposition to; however, who is permissible in colloquial English. Note here that the alternate form of this sentence, Whom did you give it to?, reads awkwardly because it ends with a preposition.
Also remember that a pronoun (or a noun) preceding a gerund* is always in the possessive case. For example:
1. She objected to their smoking in her car.
The pronoun their precedes the gerund smoking and is thus possessive.
Finally, use the reflexive pronoun only to refer to the person named in the subject of the sentence. For example:
1. Students who wish to sign up for intramural teams should speak to Ms. Smith or myself.
Incorrect: the speaker or writer is not the subject of the sentence.
Better: Students who wish to sign up for intramural teams should speak to Ms. Smith or me.
2. I forced myself to finish reading the textbook.
Correct: the reflexive pronoun myself is the object of the verb; I is the subject. Thus the same person is the subject and the object.
* A gerund is the form of a verb that ends in -ing and functions as a noun.