A pronoun is a word that takes the place of a noun. Pronouns can be in one of three cases: Subject, Object, or Possessive.
Subject pronouns are used when the pronoun is the subject of the sentence. You can remember subject pronouns easily by filling in the blank subject space for a simple sentence.
______ did the job.
I, you, he, she, it, we, and they all fit into the blank and are, therefore, subject pronouns.
Subject pronouns are also used if they rename the subject. They follow to be verbs such as is, are, was, were, am, and will be.
It is he.
This is she speaking.
It is we who are responsible for the decision to downsize.
NOTE: In spoken English, most people tend to follow to be verbs with object pronouns. Many English teachers support (or at least have given in to) this distinction between written and spoken English.
It could have been them.
It could have been they.
It is just me at the door.
It is just I at the door.
Object pronouns are used everywhere else (direct object, indirect object, object of the preposition). Object pronouns are me, you, him, her, it, us, and them.
Jean talked to him.
Are you talking to me?
To be able to choose pronouns correctly, you must learn to identify clauses. A clause is a group of words containing a verb and subject.
A strong clause can stand on its own.
She is hungry.
I am feeling well today.
A weak clause begins with words such as although, since, if, when, and because. Weak clauses cannot stand on their own.
Although she is hungry...
If she is hungry...
Since I am feeling well...
If a sentence contains more than one clause, isolate the clauses so that you can decide which pronoun is correct.
[Although she is hungry,]
[she will give him some of her food.]
[Although this gift is for him,]
[I would like you to have it too.]
To decide whether to use the subject or object pronoun after the words than or as, mentally complete the sentence.
Tranh is as smart as she/her.
If we mentally complete the sentence, we would say, "Tranh is as smart as she is." Therefore, she is the correct answer.
Zoe is taller than I/me.
Mentally completing the sentence, we have, "Zoe is taller than I am."
Daniel would rather talk to her than I/me.
We can mentally complete this sentence in two ways: "Daniel would rather talk to her than to me." OR "Daniel would rather talk to her than I would." As you can see, the meaning will change depending on the pronoun you choose.
Possessive pronouns show ownership and never need apostrophes.
Possessive pronouns: mine, yours, his, hers, its, ours, theirs
NOTE: The only time it's has an apostrophe is when it is a contraction for it is or it has.
It's a cold morning.
The thermometer reached its highest reading.
Reflexive pronouns - myself, himself, herself, itself, themselves, ourselves, yourself, yourselves- should be used only when they refer back to another word in the sentence.
I worked myself to the bone.
My brother and myself did it.
The word myself does not refer back to another word.
My brother and I did it.
Please give it to John or myself.
Please give it to John or me.
Read more at http://www.grammarbook.com/grammar/pronoun.asp