"which" v "that"

Grammar question:
When is it ok to use “that” and when should you use “which”?

“That” is defining and restrictive:
The lawnmower that is broken is in the garage.
This tells exactly which lawnmower you’re speaking of.

“Which” is not restrictive:
The lawnmower, which is broken, is in the garage.
This adds a fact about the only lawnmower in question.

Hey, imagine me giving grammar advice! Who’d a thunk it? I got hit so often on this one by MS Word that I looked it up. You only use which when you are conjoining a dependent clause modifier, otherwise use that. There are individual cases where pedants will insist other usage might be correct, but this one will keep you from getting those annoying green lines.

“We were so interested in the trip, which was very exciting, that we did not notice the time.”

Tris

Thanks for the neat definitions. Any recommendations for a good online grammar reference source?

I have trouble remembering the rules. What I do is try both “which” and “that.” If “that” sounds OK, use “that.” If “that” doesn’t sound right, use “which.” (I understand that copy editors go on “which” hunts.)