Borrowed Woe is I: The Grammarphobe’s Guide to Better English in Plain English (3rd Edition) by Patricia T. O’Conner from the library to see if I should get a copy for myself. Not five pages into the book, she solved one of the big mysteries in English for me.

Even for native English speakers, the choice between using that and which is often made by the “that sounds right” guideline. I’m all for using one’s instinct but when I put my pen to paper, that method doesn’t work.

According to Pat, this is how you know:

• Use THAT when the clause carries the point of the sentence, otherwise use WHICH
• “…[comma] THAT blah blah blah.”
  “…[comma] WHICH blah blah blah, …”