I've been doing lots of crits lately and the most common thing I noticed was the misuse of which and that. Besides the whole telling vs showing thing that's common is most manuscripts. So here's the quick and dirty and below that will be links to more in depth help with which vs that.
Basically the choice between "that" and "which" is not a choice between formal and informal as most people see it. I most usually see "that" used instead of "which" because people feel it sounds too formal. That's not the case. "Which" is a non-defining relative pronoun. While "that" is a defining relative pronoun. Which basically means "that" introduces couses that identify a person or a thing. (e.g. "He had to choose between the ten girls that had been chosen for him.") While "which" is used to give a reason or add a new fact (e.g., "I always buy his books, which have influenced me greatly."). However, be careful, when used as a pronoun "which" must have a clear antecedent--a word, phrase, clause, or sentence to which another word refers (e.g. There was a conference in Vienna, which ended on December 13th.)
Fairly simple, right? If not, please glance through the following links.
Grammar Girl
Kentworth University
Grammar Quizzes (worksheet to test your skills)
World Wide words
