Do you always have to have a comma before which?

Question

I think that you do, but I was helping a friend with some grammar and I don't want to correct it if I'm wrong...

For example, doesn't this sentence require a comma? [or change "which" to "that"]

"If we can get your lawyers to learn and apply two critical skills per workshop WHICH enable them to originate more work and get more work that is professionally rewarding, then everyone wins."

Answer 1

You're absolutely correct.

If the clause is nonessential (can be set off with commas), use the commas with 'which.'

If not, use 'that.'