Today, my family were discussing the difference between which and that. I looked it up in Strictly English by Simon Heffer. We found it very helpful. This is what he says: Perhaps the most common mistake of all with pronominal usages is the misuse of which and that as relative pronouns. It is probably not an exaggeration to say that almost everyone believes they are in most contexts interchangeable. They are not. The two sentences “the dog that was run over belonged to Mrs Smith” and “the dog, which was run over, belonged to Mrs Smith” say different things in two quite different ways. The first suggests that there were a number of dogs, and the one that happened to be run over belonged to Mrs Smith. The second supposes that there was only one dog. It happened also to be run over, which (unfortunate though it may be) is not the point of the story. The point is that it belongs to Mrs Smith. That defines; which is parenthetic, or non-defining. If one is having a conversati...