I'm doing a lot of editing lately, and I'm seeing a great deal of confusion over when to use "that" and when to use "which." I know this is a common issue, and it never used to bother me. In fact, I used to get confused too, but now that I don't, I don't want anyone else to either. So here's a simple mnemonic to help you get this right every time:
Which one? That 2?
What does it mean? Essentially it means that if there's only one of the thing in the first clause, you use which, and if there are two or more, you use that.
Jesus loves Mary Magdalene's home, which doubles as a whorehouse.
Mary Magdalene only has one home, hence you use which. "Which one"
Jesus hates the law that says he can't marry Peter.
There are many laws, certainly more than two, that Jesus could potentially hate, he hates this particular law out of many, hence, use that. "That 2"
By way of illustration, let's flip them. Let's say Mary Magdalene runs a real estate empire, and has houses all over Rome. Jesus in particular hates the Ranch style two family home near the countryside. So:
Jesus loves the Mary Magdalene place that doubles as a whorehouse, not the home near the countryside.
See? "That 2"
Meanwhile, Pontius Pilate, who believes the Jews hate our freedom and love same sex fornication, has enacted the "Sexual Freedom Law" which states that two men cannot get married. So:
Jesus hates the Sexual Freedom Law, which states that he cannot marry Peter.
See? There's only one Sexual Freedom law. "Which one."
So there you go. It's not perfect but it's good enough that I expect you to get this right every time from now on.