
Ever notice that little squiggle you get sometimes in Word when you type in “that” vs. “which”? That happens because these words are not interchangeable, strictly speaking.
Essential vs. nonessential clauses:Some modifying elements of a sentence are essential – they restrict the meaning of a modified term – while others are nonessential and don’t restrict the modified term’s meaning. These nonessential elements are set off with commas.
“That” is used in essential clauses and is not preceded by a comma. “Which” is used in nonessential clauses and is preceded by a comma.
Examples:I bought the picture frames
that were on sale.
“That were on sale” is an essential clause because the point of the sentence is that I did not just buy any frames; I bought “the frames that were on sale.” If I were to delete this clause, it would change the essential meaning of the sentence. Note that there is no comma before “that.”
We went to Harold's house,
which is just across the street.
“Which is just across the street” is a nonessential clause because I can delete it without changing the essential meaning of the sentence. Because it is nonessential, set it apart with a comma.
Bottom Line:If you leave out the element or put it somewhere else in the sentence, does the essential meaning of the sentence change? If so, the element is essential; if not, it is nonessential.