In the English language, there are rules specifying the use of "that" as opposed to the word “which”.
The general rule of thumb is that if you need a comma in the sentence to achieve your intended meaning, chances are you’re going to use "which". Commas in sentences tell the reader that there is some additional information that I the writer wish to impart. In conversational language, people will often pause momentarily after saying something to mention a trivial fact or possibly to denote sarcasm. In writing, the momentary pause is denoted with a comma. For example,
“Michael forgot to get my coat, which would have protected me from the subzero temperature.”
Here the dependent clause “which would have protected me from the subzero temperature” has served to give additional information on the coat in question. If I were saying this aloud, I would have paused momentarily before appending my little bit of “commentary.” Here’s another example:
”Rachel shot Michael in the back for eating her lunch, which had included five pounds of prime caviar and a gallon of corn whiskey.”
Again, I have used a comma followed by the word “which” to mention the contents of Rachel’s lunch. Again, notice the conversational pause in my sentence. Contrast that with the following:
“Rachel shot Michael in the back for eating the lunch that had been especially prepared for her by Queen Elizabeth’s chef.”
Here instead of pausing, I cut right to the chase. Rachel doesn’t get a lot to eat, and for Michael to guzzle her whiskey that had been especially prepared for her by the Queen’s chef was totally inappropriate. Here, I used "that" because I wanted to distinguish today’s lunch as opposed to any other lunch Rachel eats. The “that” denotes that there are a variety of lunches, but that today’s lunch is a very special lunch. It assumes that in all the multi-verses there is more than one lunch that belonged to Rachel, and that we wanted to distinguish this lunch from any other.
Let me change the sentence and see if you can spot the difference:
“Rachel shot Michael in the back for eating the lunch, which had been especially prepared for her by Queen Elizabeth’s chef.”
This sentence works too. But whereas the sentence before had been a narrowing down of categories, this sentence seeks to give additional information about the specially prepared lunch.
Again, returning to the first example:
“Michael forgot to get my coat, which would have protected me from the subzero temperature.”
as opposed to
“Michael forgot to get the coat that would have protected me from subzero temperatures.”
Compare the two and ask yourself about the slight change in meaning. If you think it’s pathetically small, you’re probably right. It gives me headaches too.
Update 3/25/02:
I seem to have stumbled upon an exception to this rule in the SAT II Writing—apparently, the College Board does not distinguish between "that" and "which." I find this interesting since the people who write the SAT II are anal retentive grammaticians, and since the rule of “that” and “which” is considered part of the foundation of stodgy, “proper” English grammar, I would have expected this to be this rule to apply in standardized testing. It seems that the only times when the SAT II will consider “that” or “which” used incorrectly are on finer points of idiom, or in cases concerning the inappropriate modifier: “the person that I met” should be “the person whom I met.”
Nevertheless, I want to mention that while the SAT II Writing does not distinguish between the two, Microsoft Word does—so if you’ve ever wondered why your usage of “which” or “that” will get a squiggle for no apparent reason, now you know.