In each set of sentences below, there will be a blank. Select the verb form that will
complete the sentence correctly. In some situations, more than one choice will be
possible, and you should indicate that with your selection. If you choose the correct
response, it might still be a good idea to consult the explanation, to see if your
explanation is the same as ours. If you look at the explanation before coming up with
the right answer yourself, we can't be responsible for the horrible consequences.
http://grammar.ccc.commnet.edu/grammar/quizzes/conditional_quiz.htm