1、提出将来会发生的事：主句：主语＋would/should/might/could＋动词原形；从句：should＋动词原形/were to＋动词原形。例句：If he should go to Harvard University, he would make full use of his time.如果他要上哈佛大学的话，他就会充分利用他的时间了。
2、提出现在的条件：主句：主语＋would/should/might/could＋动词原形；从句：动词过去式。例句：If he studied at this school, he would know the enviornment around the school very much.如果他在这所学校学习的话，他会很了解周边环境。
3、提出过去的未发生的条件：主句：主语＋would/should/might/could have＋动词过去分词；从句：had＋动词过去分词。例句： If I had read the book, I would have told you about it.我如果看过本书，我会把内容告诉你了。