“Which” or “that” – we can use both words in various contexts, but the confusion starts when we use them as a relative pronoun. Even though many people believe that the differences between those two words aren’t really differences at all, there are actually some rules for their usage. But let’s have a closer look!

Which

We use “which” for non-restrictive / non-essential clauses. This means that such clauses are not relevant or not essential to the actual meaning of the sentence, although they do add extra information. So, if you remove the non-restrictive clause, you would still understand the sentence. Keep in mind, however, to always set off these clauses by commas! 

My favorite restaurant, which served the best lobster soup in town, had closed permanently. 
(“My favorite restaurant had closed permanently.”)

The car, which has seat heating, is for sale. 
(“The car is for sale.”)

Last summer, which turned out to be the hottest one in a decade, I took my very first swimming lessons. 
(“Last summer I took my very first swimming lessons.”)

Pete’s office, which recently got renovated, is on the 7th floor.
(“Pete’s office is on the 7th floor.”)

This book, which Danielle got for her last birthday, is a great read!
(“This book is a great read!”)

💡 Non-restrictive clauses can be removed without changing the meaning of the whole sentence.

That

Compared to the usage of “which,” we use “that” for restrictive/essential clauses. Those clauses are, as the name suggests, indispensable to the meaning of the sentence since they make the mentioned object in the sentence more specific. Also, unlike non-restrictive / non-essential clauses, restrictive/essential clauses aren’t set off by commas.

Have you already seen the movie that came out yesterday?
(You’re not talking about any movie but the one that specifically came out yesterday.)

I can’t think of any other thing that can make me as happy as reading a book with a hot cup of cocoa in the evening. 
(There’s just no other thing than that.)

This is the best song that he’d heard in a while.
(It’s about a specific song in a certain time-frame.)

She ate all the cookies that were left on the plate.
(She ate all the cookies from the plate, but there might be more in the kitchen.)

The guy that you met at last night’s party is my brother.
(It implies that the other person only met one guy at the party.)

💡 Restrictive clauses cannot be removed since the sentence wouldn’t make sense anymore. 

“That” and “which” in combination

Using “that” and “which” together in a sentence is relatively rare but not unusual in higher literature, scholarly work, outdated or academic writing. 

“That” refers to a noun, whereas “which” is used as a relative pronoun for objects. In some cases, it is also possible to replace the phrase “that which” with “what.”

In a negotiating situation such as that which occurred here, it is crucial to stay focused.
(“In a negotiating situation such as the one occurring here, it is crucial to stay focused.”)

We will always remember that which they did for our country.
(“We will always remember what they did for our country.”)

The best solution is that which requires the least amount of time. 
(“The best solution is one that requires the least amount of time.”)

💡 “that which” is sometimes replaceable by “what”