These two words can sometimes be used in an either-or manner, and it won’t make much difference which one you use. However, this is not always the case, so you really need to know how and where to use each one properly.
Consider these two sentences, for example:
1 – My car that has a sliding sun roof is great to drive in the summer.
2 – My car, which has a sliding sun roof, is great to drive in the summer.
What’s the difference between them? Well, one has a restrictive clause and the other does not. Let me put that in more simple terms.
In the first sentence, the use of “that” suggests that I have more than one car, and here I am narrowing down the possibilities by describing the car as the one that has the sliding sun roof. The use of “that” here is restrictive, because I am referring to one particular car (the one with the sliding sun roof).
In the second sentence, the use of “which” means that I have included the information within the two commas as a kind of, by the way. The information (which has a sliding sun roof) is not essential to the sentence. It is therefore non-restrictive as it does not restrict the sense of the sentence in any way.
So, if you only have one car, in this example, and you want to mention that it has a sliding sun roof, then use “which”. If you have several cars, in this example, and you need to narrow down the possibilities to a single car, use “that”.
This post was prompted by a sentence I came across in an Internet marketing PDF. The sentence read in part, “… most of the products which I came across were in Ebook format.”
That is an incorrect use of “which”. If the writer was determined to use “which”, in this case, he should have written, “… most of the products, which I came across, were in Ebook format.” What the writer actually meant to say, but didn’t know, was, “… most of the products that I came across were in Ebook format.” He would have been better using the restrictive clause in this case (that).
It was not wrong to use the non-restrictive clause (which), but by using “which” he should have included the necessary commas in order for the sentence to make complete sense.
There are also cases where “that” and “which” are completely interchangeable. This happens when the use of either word forms a restrictive clause, as in these two examples:
1 – He pointed to the car that was red.
2 – He pointed to the car which was red.
If you add a comma after “which” in the second sentence, the meaning of that sentence changes. It becomes a kind of after thought: “He pointed to the car, which was red.” The car just happened to be red in color and that piece of information is not really important when you add the comma. Without the comma, the color of the car is an essential part of the information. One restricts the meaning and the other does not.