Even though it's not a popular topic, and even though I've successfully avoided grammar rants on my blog up until now, I've reached the point where I must indulge my frustration at how badly many otherwise well-educated Americans speak the English language. Allow me this one diatribe, if you please!
The relative pronoun has been abused and neglected long enough. Here's the lowdown: We have three main pronouns in English that can be used to introduce a subordinate clause: which, that, and who/whom. Here are examples of each:
The problem is that people never learn grammar in grammar school.
The grammar police, which consisted of bespectacled pencil-necks in academic regalia, strictly enforced the correct declension of pronouns.
One conjugation cop castigated a kid who had repeated the word "is" instead of using a relative pronoun.
"That" introduces a clause normally serving an adjectival function: The clause beginning with "that" either modifies a noun, or stands on its own as a subject or predicate nominative.
"Which" introduces a clause serving as an appositive. Thus, a "which" that is not preceded by a preposition should nearly always be preceded by a comma. In fact, as is the case with most appositives, the entire clause should be set off by commas. Further, the clause provides only supplemental information to the sentence, not essential information. If you are only referring to the grammar police at Harvard (not Princeton), then you may use "that", because the relative clause becomes essential to the understanding of the sentence. Here's the example: "The grammar police unit that patrols the campus of Harvard is stricter than the Grammar Gumshoes who prowl Princeton's property."
"Who/whom" introduces either type of clause, but only if the pronoun replaces a person. It is thus incorrect to say: "The boy that didn't speak English properly was punished." The boy is a "who", not a "that".
The pronoun agrees with its antecedent (whatever noun it replaces) in terms of whether to use the animate (who, whose, whoever, for whom, etc.) or inanimate (that, which, where, whatever, etc.) case, but gets its subjective or objective case (who vs whom) from its function in the relative clause. It's incorrect to say "Please return the book to whomever left it behind." While "to whom" certainly sounds right, in reality the whole clause is the object of the preposition ("to"), and "whoever" is the subject of that clause.
Don't leave your relative pronouns out! It is perfectly correct grammatically to say "The house he was turning into a meth lab burned down." However, leaving out your relatives can often cause confusion. ("He was turning into a meth lab"?!?) Moreover, relative pronoun omission leads to that annoying habit of repeating the word "is" in place of the relative pronoun: "The problem is, is people never learn grammar in grammar school." I've even heard people say "The problem being is....", which is grammatically correct, but syntactically grotesque.
Whew! I'm glad I got that off of my chest. Thanks for your indulgence: I know that sermons about verb conjugation can make one "tense".
Here's a confusing sentence that would be improved by the inclusion of a "that":
"The once-dominant Sunni minority fears the constitution as it stands, by giving Iraq's regions more powers, would deprive them of revenue from oil-rich southern and northern areas dominated by majority Shi'ites and Kurds."