Pronoun Agreement

Basic Principle: A pronoun usually refers to something earlier in the text (its antecedent) and must agree in number—singular/plural—with the thing to which it refers.

Commonly mistaken singular pronouns: The indefinite pronouns anyone, anybody, everyone, everybody, someone, somebody, no one, and nobody are always singular. This is sometimes perplexing to writers who feel that everyone and everybody (especially) are referring to more than one person. The same is true of either and neither, which are always singular even though they seem to be referring to two things.

Agreement in Gender: The need for pronoun-antecedent agreement can create gender problems. If one were to write, for instance, "A student must see his counselor before the end of the semester," when there are female students among the students mentioned, that writer will be excluding the female students, using an outdated sexist prose style. One can pluralize, in this situation, to avoid the problem:

—Students must see their counselor before the end of the semester.
Or, one could say
—A student must see his or her counselor. . . .

Too much use of "his or her" eventually becomes annoying, however, and the reader becomes more aware of the writer trying to be conscious of good form than he or she is of the matter at hand.

Trying to conform to the above rule (#2) can lead to a great deal of trouble. It is widely regarded as being incorrect to say

—Somebody has left their bag on the floor.

The language may eventually change to accommodate this violation of pronoun agreement as a way to avoid the "his or her" or the sexist "his" only. There is a great deal being said, however, for using the word "their" as the gender-non-specific, singular pronoun. and you can read all about it at the The University of Texas, where a web-site has been dedicated to the use of "their" in this way in the writings of Jane Austen, William Shakespeare, and other literary figures. Another site dedicated to the "gender-free pronoun" is at Gender-Neutral Pronoun Frequently Asked Questions.

Who or Whom: One of the most frequently asked questions about grammar is about choosing between the various forms of the pronoun who: who, whose, whom, whoever, whomever. The number (singular or plural) of the pronoun (and its accompanying verbs) is determined by what the pronoun refers to; it can refer to a singular person or a group of people:

—The person who hit my car should have to pay to fix the damages.
—The people who have been standing in line the longest should get in first.

It might be useful to compare the forms of "who" to the forms of the pronouns "he" and "they." Their forms are similar:

To choose correctly among the forms of "who," re-phrase the sentence so you choose between "he" and "him." If you want "him," write "whom"; if you want "he," write "who."

—Who do you think is responsible? (Do you think he is responsible?)
—Whom shall we ask to the party? (Shall we ask him to the party?)
—Give the box to whomever you please. (Give the box to him.)
—Give the box to whoever seems to want it most. (He seems to want it most. [And then the clause "whoever seems to want it most" is the object of the preposition "to."])
—Whoever shows up first will win the prize. (He shows up first.)

The number of people who use "whom" and "who" wrongly is many. The problem is a difficult one and it is complicated by the importance of tone, or taste. Take the common expression, "Whom are you, anyways?" That is of course, strictly speaking, correct—and yet how formal, how stilted! The usage to be preferred in ordinary speech and writing is "Who are you, anyways?" "Whom" should be used in the nominative case only when a note of dignity or austerity is desired. For example, if a writer is dealing with a meeting of, say, the British Cabinet, it would be better to have the Premier greet a new arrival, such as an under-secretary, with a "Whom are you, anyways?" rather than a "Who are you, anyways?"—always granted that the Premier is sincerely unaware of the man's identity. To address a person one knows by a "Whom are you?" is a mark either of incredible lapse of memory or inexcusable arrogance. "How are you?" is a much kindlier salutation.
James Thurber Ladies' and Gentlemen's Guide to Modern English Usage

The only problem most writers have with "whose" is confusing it with who's, which looks like a possessive but is really the contraction for "who is." In the same way that we should not confuse his with he's (the contraction for he is or he has), we should not confuse whose with who's.

—Who's that walking down the street?
—Whose coat is this?
—I don't care whose paper this is. It's brilliant!

"Whose" can be used to refer to inanimate objects as well as to people (although there is a kind of folk belief that it should refer only to humans and other mammals): "I remember reading a book—whose title I can't recall right now—about a boy and a basenji."

The content of this page is taken from this web site: http://webster.commnet.edu/grammar/pronouns.htm