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When do I use which or that in a sentence?

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Let me give you two examples to explain the correct usage of "which" and "that."

1. We recently installed new siding on our house, which was flooded last month.

Use "which" when the information in your subordinate clause ("which was flooded last month") is non-essential to the meaning of the sentence. If you took away the subordinate clause, the reader would still know what house you are referring to.

2. I returned the book that I bought last night.

You should use "that" when the information directly following it is essential to understanding the sentence. Without "that I bought last night," the reader wouldn't know which book you are talking about.

The way I remember it is by thinking of "that" as a tight knot within a sentence and "which" as a looser bow.

Do you sometimes wonder whether to use that or which in a sentence? In many cases, in British English, both words are equally correct.

√ She held out the hand which was hurt.

√ She held out the hand that was hurt.

In these sentences, that and which are introducing what’s known as a restrictive relative clause. This is a clause containing essential information about the noun that comes before it. If you leave out this type of clause, the meaning of the sentence is affected – indeed, it will probably not make much sense at all. Restrictive relative clauses can be introduced by that, which, whose, who, or whom.

The other type of relative clause is known as a non-restrictive relative clause. This kind of clause contains extra information that could be left out of the sentence without affecting the meaning or structure. Non-restrictive clauses can be introduced by which, whose, who, or whom, but you should never use that to introduce them. For example:

A list of contents would have made it easier to steer through the book, which also lacks a map.

She held out her hand, which Rob shook.

Note that a non-restrictive clause is preceded by a comma (so as to set off the extra information), whereas no comma should precede a restrictive clause (indicating that the information is essential, not extra):

I bought a new dress, which I will be wearing to Jo's party. [non-restrictive]

I was wearing the dress that I bought to wear to Jo's party. [restrictive]

When I attended the University of Phoenix, we had a grammar checker called WritePoint.  Whenever I used the word "which" in a sentence, it would make a note explaining in scholarly writing, "that" was preferable to "which," and if "that" could be substituted for "which" without changing the meaning of the sentence, replace "which" with "that." 

Absolutely! Also keep in mind that you don't necessarily always use a comma when using "which", so keep your sentence structures in mind!

I agree with Rebecca and resound with her conclusion that even though it seems random, that and which are consistent in sentences. Simply, if you are doubting the use of that or which... remember that a "a witch is a ditch". (once you understand that, it'll stick!)


That's a great way to remember it Jonathan!

"A which is a ditch" -- Beautiful!