Anyone who tries to explain
"that" and "which" in less than an hour is asking for trouble. Fowler, in
his Modern English Usage, takes 25 columns of type.
-William Zinsser, On Writing Well
One summer long ago my husband and I engaged in a personal grammar contest (for sheer sport, of course). We took perverse delight in spotting each other's grammatical faux pas, but the contest also revealed some unflattering grammatical weaknesses and misconceptions. Chief among them was the correct use of which, that, and who in restrictive and nonrestrictive clauses. At least we were in good company. Even essayist E.B. White blundered here, contrary to his own advice to others (see Strunk and White's Elements of Style). And, contrary to Zinsser, I think it is possible to address that and which in less than one hour.
at some examples:
The wagon, which [incidentally] is now broken, was purchased at a home improvement store.
The clause which is now broken can be omitted without disrupting the meaning of the sentence. It is not essential to the sentence (nonrestrictive). It's simply additional information.
The wagon that is broken was purchased at a home improvement store.
This one particular wagon is broken; others are not broken. The clause that is broken restricts the meaning of wagon to the one that is in disrepair (restrictive).
The brochure, which was designed by our marketing department, won high praise at the meeting.
The nonrestrictive clause which was designed by our marketing department provides parenthetical information and can be omitted without destroying the meaning of the sentence.
The brochure that was designed by our marketing department won high praise at the meeting.
The marketing department brochure was a winner; the brochures designed by other departments did not win kudos.
The attorney, who graduated from Yale, filed the motion with the court yesterday.
The clause adds parenthetical information (nonrestrictive).
The attorney who graduated from Yale filed the motion with the court yesterday.
It was specifically the Yale graduate who took action rather than the Harvard graduate (restrictive).
The teachers, who have educated my son, deserve an award for patience.
This nonrestrictive clause refers to all of the teachers your son has had in his school career.
The teachers who have educated my son deserve an award for patience.
You are now referring to only the more effective of your son's teachers; other teachers may not have had an impact (and were probably not as patient with your son).
are several reasons for keeping these grammatical distinctions in mind.
First, if you as the author can't decide how important or how defining a clause is in a sentence, then your readers will be confused as well. Is the clause essential to the meaning of the sentence (restrictive), or is it a parenthetical remark (nonrestrictive)? Decide before you write. Stop, rethink, and reconstruct the sentence to avoid confusion or misinterpretation.
Second, everyone occasionally writes a sentence that has two different interpretations. The habits of correctly using that and which and of setting off nonrestrictive clauses with commas (or in parentheses) can save us from ambiguity even when we are unaware of the possibility.
Finally, the word that is more common in speech than which. As a result, we perceive which as more formal than that. It isn't, but it does indeed seem so. Sometimes a sentence has two clauses, and we feel that a that-which combination scans better than a that-that or a which-which combination. Your options are to recast the sentence, to live with the repetition in the sentence, or to ignore the rule for the sake of readability.
If you're interested in a more extensive discussion of the which/that/who conundrum, take a look at Diana Hacker's A Writer's Reference (print, 5th edition) and its companion website. Simply click on Language Debates, and select that versus which. For the grammar-obsessed, I recommend Otto Jespersen's The Philosophy of Grammar (University of Chicago Press, 1924).
Do you have a grammar question? Comments? Suggestions? Please let me know.