To understand when to use which and that, we first need to understand the idea of
defining and non-defining relative clauses.
Non-defining relative clauses add EXTRA information to the sentence.
Defining relative clauses add ESSENTIAL information to the sentence.
Here’s an example. Let’s imagine that it’s Friday, and I say:
▶️ The bananas that I bought on Monday are rotten.
▶️ The bananas, which I bought on Monday, are rotten.
In the first sentence, it’s possible that we have two types of bananas in the house:
▶️ Older bananas – bought on Monday
▶️ Newer bananas – bought on Wednesday
…and that only the older bananas are rotten, but the newer bananas are not rotten.
This means that the phrase “that I bought on Monday” is essential information (a
defining clause), because it tells us which group of bananas we are talking about.
In the second sentence, all the bananas in the house were bought on Monday, and
they are all rotten. In this case, “which I bought on Monday” is extra information (a
non-defining clause).
Use that with defining clauses (essential information).
Use which with non-defining clauses (extra information) – and use a comma
before it.
▶️ This is the book that I want to buy.
(“that I want to buy” is essential because it defines the book we are talking
about)
▶️ This book, which I bought recently, is very good.
(“which I bought recently” is extra information; it is not essential to the main
point of the sentence – This book is very good)