Mike Stevens wrote to ask about something that’s bugged him for quite a while.
“When is ‘that’ used, and when is ‘which’ the more appropriate word?”
In general, newspaper style is to use “that” to introduce an essential clause and “which” to introduce a nonessential clause. An essential clause can’t be removed from a sentence without changing meaning; a nonessential clause can be, but it’s included to give more information about the subject.
The meaning dictates whether you need an essential or nonessential clause. I learned the rule with this example:
Essential clause: Nike shoes that fall apart should be returned for a refund. (Only those few Nike shoes that fall apart should be returned.)
Nonessential clause: Nike shoes, which fall apart, should be returned for a refund. (You could take out the two commas and the “which fall apart,” and the sentence would read “Nike shoes should be returned for a refund.” It’s a complete sentence all right, but it’s also maybe libelous. In this case, the writer is declaring that all Nike shoes fall apart. The newspaper that publishes such a sentence will need good lawyers.)
Stevens asked about these sentences:
* “I took the book that was on the table.” Correct. Which book was taken? The one that was on the table. The clause beginning with “that” is essential to understanding which book we’re talking about.
* “I took the book which was on the table.” No, but you could write, “I took the book, which was on the table.” The clause beginning with “which” is nonessential, but it gives more information about the book that was taken.
By the way, some grammar books used to teach (and may still teach) that these are “restrictive” and “nonrestrictive” clauses. Same thing; different name. Try not to let the terms scare you. It’s more important to be able to apply a rule than to know all the things it’s called.