In writing, small things can make the difference between a professionally adept manuscript and an average manuscript.
Not knowing when to use “its” versus “it’s” or “there” versus “their” are two common errors made by novice writers. But knowing when to use “which” or “that” in a sentence is often a challenge for even the most experienced writer.
Here are a few pointers to help you determine when to use each one, and why.
Struik and White’s “The Elements of Style” has this to say about “which hunting”:
Which is a non-defining, or non-restrictive, pronoun, while that is a defining, or restrictive, pronoun.
They explain further:
Which introduces a non-restrictive clause, while that introduces a restrictive clause.
A non-restrictive relative clause is one that does not serve to identify or define the antecedent noun. By contrast, a restrictive clause is not parenthetic and serves to define the antecedent noun (that is, the noun that comes before the defining clause). A sentence containing a restrictive clause cannot be split into two independent statements.
Is that a bit clearer now? Not really? Okay, let’s look at an example.
He ate the apple, which was the biggest in the bowl, and asked for more.
He ate the apple that was the biggest in the bowl and asked for more.
In the first sentence, the dependent clause "which was the biggest in the bowl" is non-restrictive. The information is not crucial to the meaning of the sentence. If you leave it out, you're left with "He ate the apple and asked for more" and that's what the sentence means.
In the second sentence, however, the dependent clause "that was the biggest in the bowl" is important, because it defines the apple as the biggest in the bowl. Perhaps there are other apples in the bowl; the one described in this sentence is the biggest one. If you leave out "that was the biggest in the bowl," you won't know which apple is being referred to and why. Why did he eat the biggest apple and still ask for more? Is the "he" eating the apple a greedy person or just a very hungry one?
Put another way:
Which is used to give the reader non-essential information and introduces a clause contained within a sentence that can stand alone without the information.
That is used to tell the reader essential information and introduces a clause without which the sentence cannot stand alone.
A concrete example:
The groceries, which are in the shopping bag, need to be packed away. The groceries that are in the shopping bag need to be packed away.
See the difference? In the first sentence, it's not vital to know that the groceries are in the shopping bag, just that they need to be packed away. In the second sentence, it's important to know that it's the groceries in the shopping bag that need to be packed away and not, say, the groceries in the car or on the kitchen shelf.
Here's a final little tip on "which" versus "that":
Which appears with a comma; that doesn't.
If you’re looking for more detailed explanations of how to hunt a “which”, browse the following sources:http://www.grammarbook.com/grammar/whoVwhVt.asp
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