He picked the toy that was broken.
Dogs that bark are too noisy.'That' introduces a 'restrictive relative clause'. Before your eyes start glazing over, this means that without this clause the sentence doesn't make sense. Think about it. "He picked the toy." Which toy? A restrictive clause defines a specific part of the sentence, too. If we look at "Dogs that bark are too noisy", 'that bark' is restricting the sentence to a certain type of dog: the ones that bark. Without 'that bark' you have the sentence: Dogs are too noisy. This means you think all dogs are too noisy, not just the ones that bark. Now, imagine that you wish to condense the following sentences into one:
The toy was blue. It was removed from the shelf.
The toy that was blue was removed from the shelf.Here you are defining for your reader which toy was removed: the blue one. Or...
He picked a toy. It was broken.
He picked the toy that was broken.Here you are defining for your reader which toy was picked: the broken one. Note that you never use a comma before 'that' in the case of a restrictive clause. Now we come to the 'non-restrictive relative clause'. This defines when you use 'which'. However, once you understand the difference between restrictive and non-restrictive you won't forget it, and you'll choose the correct word every time. A non-restrictive relative clause introduces a part of the sentence that could be left out without affecting the structure or meaning of the sentence. And here you would use 'which' (or 'who', 'whose' or 'whom'), not 'that'. For example:
He picked up the toy, which had a broken wheel, and examined it.
She jumped up and down on the bed, which had squeaky springs, until her head hit the ceiling.
The trees, which were green and lush, grew at the back of the garden.Always use a comma before your non-restrictive clause. This defines your clause as extra information in the sentence. I hope this article has helped you. I'm happy to help you too. If you would like me to write, edit or proof-read any work for you, do contact me.