What is the difference between the words which and that?
For example I have the following sentences:
“I have a car which is blue.”
“I have a car that is blue.”
Are there any rules specifying usage of which and that?
回答：There is no difference in meaning. There is a difference in use.
Relative clauses—the sort of clause you use, “which is blue” / “that is blue”, which tells us something more about the noun referred to by which or that—are of two sorts: restrictive and nonrestrictive.
A restrictive clause restricts the noun it modifies to what’s defined in the clause. The clause identifies the noun and is essential to your meaning. For instance:
I looked at the books which he sent me last week. … The books I’m talking about are the ones which he sent me last week.
A nonrestrictive clause adds information about the noun it modifies. The clause is almost parenthetical, it could be left out without changing your meaning:
I looked at the books, which he sent me last week. … The books have already been identified in our discourse, I’m just throwing in a by-the-way comment about when I got them.
Note the comma in that sentence: it sets the clause off and ‘marks’ it as something added. Now Which/That …
That may only be used at the head of a restrictive clause. It is not used, in any register, with a nonrestrictive clause.
Which may be used at the head of either sort of clause.
You may encounter another rule, which is loudly disputed. About a century ago the Fowler brothers suggested a “division of labour”—using that only at the head of restrictive clauses and which only at the head of nonrestrictive clauses. This proposal made sense to many people, it was picked up by several prestigious grammarians and style guides, and in consequence it’s often cited as a “rule”.
But the fact is, this division has never been generally adopted, and there’s no reason to follow it. I myself don’t follow it; quite the opposite, I employ “which” wherever I can, because I believe that that has entirely too much work to do already and a multiplicity of thats is likely to confuse the reader.
But that’s my choice. You’re free to follow your own rule, as long as you don’t put that at the head of a non-restrictive clause.