Differences between “that” and “which”

A relative pronoun is a pronoun that marks a relative clause within a larger sentence. It is called a relative pronoun because it relates to the word that it modifies and they often do so as clauses rather than as single, descriptive words.

Here is an example (the clauses are in bold):

• Bring me the basket.

• Bring me the basket that has vegetables in it.

• The basket, which is brown, has vegetables in it.

In the second sentence, if you removed the clause “that has vegetables in it,” the meaning of the sentence would be lost. Nobody would know which basket the speaker wants so the clause is necessary to keep the meaning of the sentence. 

In the third sentence, if you removed the clause “which is brown”  the meaning of the sentence would not be lost so they would still know that the basket has vegetables in it.

The difference between the use of the words “that” and “which” as relative pronouns depends on whether the clause they belong to is necesary or not to the meaning of the sentence.