Explain the difference between 'that' and 'which' in relative clauses.

In relative clauses, both 'that' and 'which' are used to introduce additional information about a noun or pronoun in a sentence. However, there are some differences in their usage and meaning.

1. Restrictive vs. Non-restrictive clauses:
- 'That' is used to introduce restrictive relative clauses. These clauses provide essential information about the noun or pronoun they modify, without which the meaning of the sentence would be incomplete or ambiguous. Restrictive clauses are not set off by commas.
Example: The book that I bought yesterday is very interesting. (Here, the relative clause "that I bought yesterday" restricts the meaning of the noun "book" by specifying which book is being referred to.)
- 'Which' is used to introduce non-restrictive relative clauses. These clauses provide additional, non-essential information about the noun or pronoun they modify. Non-restrictive clauses are set off by commas.
Example: The book, which I bought yesterday, is very interesting. (Here, the relative clause "which I bought yesterday" adds extra information about the book, but it does not restrict or limit the meaning of the noun "book.")

2. Prepositions:
- 'That' cannot be preceded by a preposition. Therefore, when the relative clause requires a preposition, 'which' is used instead.
Example: The house in which I grew up is now for sale. (Here, the preposition "in" requires the use of 'which' instead of 'that'.)

3. Verb omission:
- 'That' can be used to introduce a relative clause where the verb is omitted. This is common in informal speech or writing.
Example: The car that I want to buy. (Here, the verb "want" is omitted, but it is understood that the relative clause refers to the car that I want to buy.)

4. Informal vs. Formal usage:
- 'That' is generally used in both formal and informal contexts.
- 'Which' is more commonly used in formal writing or when the relative clause is non-restrictive.

It is important to note that in some cases, 'that' and 'which' can be used interchangeably, especially in non-restrictive clauses. However, it is crucial to consider the context and intended meaning to choose the appropriate relative pronoun.