The words "that" and "which" are both relative pronouns, which means they are used to introduce clauses that provide additional information about a noun in a sentence. However, there are some key differences between the two words that are important to understand in order to use them correctly in your writing.
One of the main differences between "that" and "which" is that "that" is used to introduce clauses that are essential to the meaning of the sentence, while "which" is used to introduce clauses that are nonessential or supplementary. In other words, the information provided by the clause introduced by "that" is necessary for the reader to fully understand the sentence, while the information provided by the clause introduced by "which" is not essential and can be removed without changing the basic meaning of the sentence.
For example:
"The book that I read last night was really interesting."
In this sentence, the clause "that I read last night" is essential to the meaning of the sentence, because it specifies which book the speaker is referring to. Without this clause, the sentence would not make sense. Therefore, we use "that" to introduce the clause.
On the other hand:
"The book, which I found at the library, was really interesting."
In this sentence, the clause "which I found at the library" is nonessential and provides additional information about the book. The sentence still makes sense without this clause, so we use "which" to introduce it.
It's also important to note that "that" is often used without a comma, while "which" is usually preceded by a comma. This is because the clauses introduced by "which" are usually nonessential and can be set off with a comma for clarity. However, there are some situations where "which" can be used without a comma if the clause is essential to the meaning of the sentence.
In summary, "that" is used to introduce clauses that are essential to the meaning of a sentence, while "which" is used to introduce clauses that are nonessential or supplementary. Understanding the difference between these two words and using them correctly can help you write more clearly and effectively.