When is it proper to use "which" and when is it proper to use "that"
A propriety that is fast fading from usage in the hectic pace of modern language evolution:
'Which' is for non-restrictive clauses ('This is my cat, which has matted fur') while 'that' is for restrictive clauses ('This is the cat that has matted fur'). Nowadays, 'which' is used in both types of clauses, but 'that' should still be restricted to restrictive ones.