That versus which
A lot of second language speakers have trouble with this, so today I'd like to direct anyone who feels uncertain about it to listen to this very clear post by editor Kory Stamper.
Merriam Webster's "Ask the editor" is where you'll find her incisive and enlightening remarks.
The bottom line is this. Although the "rule" concerns restrictive and non-restrictive clauses, actual usage is not as strict as the rule.
We can say the idea that I had a minute ago (restrictive clause) or the idea which I had...
In the case of non-restrictive clauses, though, we do need to restrict our use to which, bracketed by commas.
Like this:
The rule about always using that for restrictive clauses, which was made by grammarians, is not necessarily followed by all native speakers of English.