When do you use "that" and "which"?
"That" introduces essential clauses, while "which" introduces nonessential clauses.
What’s an essential clause? Essential clauses don’t have commas surround them, while nonessential ones are surrounded by commas.
I do not like toys that are not safe for children.
The toy, which has sharp edges, isn’t safe for children.
If you have already used "that", "this", "these", you can follow up with "which" for both essential and nonessential clauses.
This is an opportunity which you must treasure.
This opportunity, which was created by your mentor, is rare to come by.