I used to have a big problem with the choice between ‘that’ and ‘which’. Looking at many articles and advertisements, it seems I’m not alone – many writers and professionals regularly get this one wrong. A common problem seems to be overusing ‘which’ in place of ‘that’, perhaps because it seems more classy and formal. Or perhaps people think there’s no difference, and it’s just a matter of preference. Unfortunately, it’s not – one or other is always right, and they are never interchangeable.
Some years ago, an assistant editor (who I was supposed to be training) helped me out with ‘that defines, which describes’. This concisely sums up the difference: ‘that’ introduces an essential definition of the subject, while ‘which’ introduces an optional description of it. The following two sentences illustrate this nicely:
He stopped the first car that was driven by a woman.
He stopped the first car, which was driven by a woman.
In the first sentence, ‘that’ introduces an essential definition. Without the rest of the sentence, the meaning is completely different. But in the second, ‘which’ introduces a description, without which the sentence would still have the same meaning.
Note also that ‘which’ always follows a comma, while ‘that’ runs straight on. The comma denotes a pause in speech, as you can confirm by reading out the two examples above. So you can always say your sentence out loud, see if a pause is needed, and make your selection on that basis. (If you pause, there’s a comma and therefore it’s ‘which’.)
Finally, if you use Microsoft Word (doesn’t everyone?) you can simply turn on its live grammar check feature and it will highlight your that/which howlers (with green wavy underlining). I’ve just tried it on the examples above and it worked – in fact, this is one of the few areas where it’s reliable pretty much all of the time. But it can get confused with longer, more complex sentences, so it pays to know the rule yourself.