Discussion Forum—A Way with Words, a fun radio show and podcast about language

Hello,

I have never quite resolved the issue of when to use “which” and when to use “that”. For example, should I say, “The book that he wrote was very good.”, or should I say, “The book, which he wrote was very good.”? I've checked an English grammar style book, which I can't find at the moment (or should it be “that I can't find at the moment”?), but it's still not clear to me. Are there some hard and fast mechanical rules regarding this issue?

Thanks,
Ray Mitchell