That vs which

When do you use which, and when do you use that?

As an English person, I did not think there was a right or wrong answer to this question. But after listening to The Modern Scholar: A Way With Words Part III: Grammar for Adults, I now know there IS an answer to this question.

That
Use 'that' when the clause identifies or determines a noun phrase.

Example 1: This is the suit that I wore on my wedding day.

In this case, the suit is identified by the clause "that I wore on my wedding day." By saying? "that I wore on my wedding day," everyone will know exactly which suit I'm talking about.

Which
Use 'which' when the clause adds optional information about the noun phrase.


 * This is the suit which I love and that I wore on my wedding day.

Here "which I love" is optional. It gives extra information, but it does not help identify which suit I'm talking about.