If you had diagramed as many sentences as I did way back when, you wouldn’t need this quick & dirty lesson, but here goes.
Consider the difference in meaning in this sentence when “that” is interchanged with “which”:
Tigers that have stripes are an endangered species.
(ie, only those tigers with stripes are endangered)
Tigers, which have stripes, are an endangered species.
(ie, all tigers are endangered – and for clarification/descriptive purposes, they all happen to have stripes, too)
“That” introduces restrictive or defining clauses (no commas used), while “which” introduces nonrestrictive (albeit informative) clauses (set off with commas).
And for those of you youngsters who have no idea what a sentence diagram is … perhaps this omission in your education explains why I can take apart your grant narratives and identify the missing pieces & logic so easily. Thank you, Mr. Bickford, for forcing me to think about language with such clarity & break down so many complex sentences into those straight, slanted, and curving lines.