We have several relative pronouns, which can be confusing.

Use ‘who’ and ‘that’ for people and use ‘which’ and ‘that’ for things.

“There is the fireman who rescued my cat.” “This is the fireman that rescued my cat.” They’re both grammatically correct but it’s generally considered more polite to use ‘who’ when referring to people.

“This is the car that does an amazing 100 miles to the gallon.” “This is the car which does an amazing 100 miles to the gallon.” Both correct. Choose whichever you prefer as both imply that this car is the one with the amazing fuel consumption.

However, putting a comma before ‘which’ usually indicates an extra piece of information.

“This is the car, which does an amazing 100 miles to the gallon” means “this is the car (I was talking about earlier) and it also happens to do an amazing 100 miles to the gallon”.

The comma before ‘which’ in my opening sentence is doing the same thing – giving additional information.

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