I did some proofreading for one of my clients recently and was asked: When should we use ‘which’ and when should we use ‘that’?”
The two words are used interchangeably quite often but the following example should help you when you need to decide on the better choice of grammar. (I hesitate to say ‘correct grammar’ as there are so many opinions and authorities out there.)
“This is the stone that I picked up when walking the dog.”
This stone, which I picked up when walking the dog, is agate.”
As you see, ‘which’ introduces an explanatory clause.