Many people struggle with when to use ‘that’ and when to use ‘which’. To help me remember when to use each, I like to keep it really simple.
A suitcase that has lost its handle is useless.
In this sentence, the word ‘that’ introduces essential information about the suitcase. If you leave it out, the meaning is changed.
The suitcase, which had no handle and was covered in airport stickers, dragged behind him.
In this sentence, the word ‘which’ introduces extra information about the suitcase. If you leave it out, the sentence is less descriptive, but its general meaning is not changed.
Use a comma before and after extra information (imagine you’re putting it in brackets):
The leaf, which was a delicate shade of green, stuck to the window.
Essential information does not need commas:
Use only leaves that are green for garnishing the cocktails.
Kiwifruit that were affected by disease had to be binned.
Kiwifruit, which are usually expensive at this time of year, were on special today.
Cars that have hybrid technology are really fuel-efficient.
My car, which is a hybrid, goes 100km on 6 litres.
The biggest savings came from items that were discounted.
The biggest saving came from chicken, which fell about 20 cents per kilo.
The Oxford English Dictionary says it’s also correct to use ‘which’ to present essential information. Be aware that other people may follow this rule. I stick to one word for each situation because I find that easiest to remember.