Relative Pronouns introduce two types of clauses:
An adjective clause:
In the following sentence, we have an example of an adjective clause introduced by a relative pronoun:
My friend, who was always there for me, proved always faithful.
The adjective clause meets the criteria of a clause (it has a subject and a verb) and the criterion of an adjective (it modifies the noun friend).
A noun clause:
In the following sentence, we have an example of a noun clause introduced by a relative pronoun:
My friend saw who was driving the car.
The noun clause also meets the criteria of a clause. It meets the criterion of a noun because it is the object of the verb saw.
Punctuation Rules for Relative Clauses
Note in the example “My friend, who was always there for me,” that the relative clause is separated from the main clause by commas. This is an example of a nonrestrictive relative clause. This means that the full understanding of the sentence (and its subject friend) is not dependent on the relative clause. It is, therefore, separated by commas.
On the other hand, in the sentence “He had many friends who deserted him when the going got rough," the relative clause is restrictive. The relative clauses restricts or adds fuller meaning to the the main clause (and its subject friends). It is not separated by commas.
Two more examples:
The man, who was the murderer, was convicted. (Nonrestrictive)
The person who committed the crime was unknown. (Restrictive)
My younger sister, who lives in North Carolina, is Vicki. (Nonrestrictive)
My sister that lives in North Carolina is Vicki. (Restrictive)
Case Rules for the Relative Pronoun “Who” (and its Derivatives)
The relative pronoun can be the subject or the object of its verb, and, as such is, either nominative (who) or objective (whom). Take a look at the following examples:
The jury convicted the man who committed the murder. (The subject of the relative clause is who. It is in the nominative case.)
The witness said that he saw Mr. Jones, whom the murderer shot. (The subject of the relative clause is murderer. The object of the verb shot is whom.)
Finally, a trick question:
Which of the following sentences is correct?
I will award the prize to whomever does his best.
I will award the prize to whoever does his best.
Whoever (a derivative of who) is the correct choice. The relative pronoun whoever is not the object of the preposition to; the entire phrase (whoever does his best) is. Whoever is the subject of the clause and belongs in the nominative case.
More Information on Relative Pronouns
The Owl at Purdue - Relative Pronouns
For information on how reflexive pronouns differ from other types of pronouns, see The English Personal Pronoun System for ESL Students