I often see “which” where “that” is appropriate.

Problem:
The pronoun “which” is not interchangeable with the pronoun “that”.

Explanation:
Some speakers and writers tend to favor “which” over “that” because they believe that “which” sounds more elegant or sophisticated than “that”. Unfortunately, most of the time they should use “that” instead of “which”.

When used to introduce a clause, these two pronouns are not interchangeable. Only one of these examples is correct: Nobody likes a cat that scratches. vs. Nobody likes a cat which scratches. The first example is correct.

One way to remember which pronoun to use (no pun intended) is to see whether you can put the clause inside commas. You would not write Nobody likes a cat, which scratches. because you need the scratching behavior to identify the cat. The comma in this example tells the reader that the clause is unnecessary, which is not your intent. The clause is necessary to identify the cat. You should instead write Nobody likes a cat that scratches. (notice the lack of commas).

Here are two more examples, and each shows correct use of “that” or “which”:

• Joe’s car, which could go from zero to sixty miles per hour in three seconds, won the contest for the fastest car on the drag strip.
• The car that won the contest for the fastest car on the drag strip was Joe’s car.

Notice that we can remove the “which” clause and its commas without destroying the significant point of the first example: Joe’s car won the contest for the fastest car on the drag strip.

Solution:
Use “that” when when you must keep the (“that”) clause to maintain the point of the sentence. Use “which” when you can maintain the point of the sentence after dropping the (“which”) clause. Also, make sure that “that” clauses do not go inside commas and that “which” clauses do go inside commas.