That = Which

THAT = WHICH (WITHOUT A COMMA)

Here is a quick summary of when to use a comma with which (and who):

Comma After. If the clause (shown in bold below) is required to identify whatever it follows (car in this example), then there are no commas.

The car which I drove on Tuesday has been sold.

The car, which I drove on Tuesday, has been sold.

Use That If You Want. If you think the word 'that' sounds better than 'which', then use 'that' (provided there are no commas).

The car that I drove on Tuesday has been sold.

No Comma After. If the clause is just additional information, because whatever it follows has already been identified (John's red Mustang in this example), then use commas.

John's red Mustang, which I drove on Tuesday, has been sold.

GRAMMAR CHECKERS WON'T HELP ON THIS ONE

Grammar checkers do not understand what has been written. They perform a 'mathematical' grammar check on writing. This is why they are rubbish at determining whether there should be a comma before which or who. Many checkers encourage you to use 'that', and if you don't, they offer you the with-comma version. This relies on your knowledge or instinct to get it right. It may also cramp your style as it does not offer the non-comma version.