Why do we use ''which'' and ''that'' in a sentence?

Answer 1

The word 'which' is an adjective and a pronoun.

• The adjective 'which' is placed before a noun to describe that noun.

The pronoun 'which' is a relative pronoun and an interrogative pronoun.

• The relative pronoun 'which' introduces a relative clause, giving information about its antecedent.
• The interrogative pronoun 'which' introduces a question.

The word 'that' is an conjunction, an adjective, an adverb, and a pronoun.

• The conjunction 'that' is used to join clauses or sentences.
• The adjective 'that' is placed before a noun to describe that noun.
• The adverb 'that' is used to modify an adjective.

The pronoun 'that' is a demonstrative pronoun and a relative pronoun.

• The demonstrative pronoun 'that' takes the place of a noun indicating distance in place or time.
• The relative pronoun 'that' introduces a relative clause, giving information about its antecedent.

EXAMPLES

• I can't decide which hat to wear. (adjective, describes the noun 'hat')
• I am writing about a computer which doesn't boot up. (relative pronoun, gives information about its antecedent 'computer')
• Which is the best route to the capital? (interrogative pronoun)
• We were glad that you were able to come. (conjunction)
• I enjoyed that movie. (adjective)
• We use the air conditioner when it gets that hot. (adverb, modifies the adjective 'hot')
• That is a good book. (demonstrative pronoun)
• There's the lady that used to live next door. (relative pronoun, gives information about its antecedent 'lady')

Answer 2

"We use 'which' to provide additional information in a nonrestrictive clause, which is information that can be omitted without changing the core meaning of the sentence. 'That' is used to introduce a restrictive clause, which provides essential information that cannot be omitted without changing the core meaning of the sentence."