A. Well, we use "that" when we want to restrict our subject to some smaller subset, by using the descriptive phrase that follows that. Hence:
Please pick up the clothes that you dropped off at the dry cleaners yesterday.I don't just want you to pick up any old clothes; I want you to pick up the ones you dropped off yesterday. No other clothes will do. Hence, I use the restrictive "that" to tell you exactly what clothes I want you to pick up.
Compare this with the correct unrestrictive usage of "which":
When I was a junior in college I drove a lime-green 1977 chevy, which I bought with the pittance I earned in my summer job with Greenpeace.The clause following "which" doesn't tell us information we have to have in order to know which car it is. Even if we don't know that you spent your summer haranguing people to wear hemp shoes, we would still know exactly what car we're talking about: your car, the unfortunately colored American gas-guzzler that leaks oil and parallel parks about as well as the Love Boat. The information provided by our descriptive clause is interesting, but not necessary for us to identify its subject.
