(从句)If + were/did(动词的过去式)，(主句)主语 + would/might/should/could + do(表示对现在情况的假设)
If I were you, I would not be so proud. 如果我是你，我不会如此自负。
I don’t have a cellphone. If I had one, it would be convenient for me to get in touch with others.
If I were in your position, I would think better of it. 如果我处在你的位置，我会好好考虑它。
(从句)If + had done, (主句)主语+ would/might/should/could +have done(表示对过去或已经发生事情的虚拟假设)
What a pity it is that you didn’t attend the concert yesterday! If you had attended the concert, you would have seen the famous singer.
Anyone in his position would have done the same.
=If anyone had been in his position, he would have done the same. 任何处在他位置的人都会这样做的。
(从句)If + were/did(动词过去式)/were to do/should do，(主句)主语+ would/might/should/could + do(表示对将来的假设)
If he should refuse (= If he were to refuse=If he refused), it didn’t matter at all.
If you shouldn’t pass the college entrance examination, what would you do?
在虚拟条件句中，如果出现有were, had, should，可以省去if, 把这些词放在句子前面，构成虚拟倒装句。
Should he act like that again, he would be fined. 如果他还这样做，就要受罚。
Had the doctor come in time last night (=If the doctor had come in time last time), the boy would have been saved. 昨天晚上要是医生及时到达，小孩就会得救。
Were I to go to the moon one day, I would see it with my own eyes.
Had I enough money (= If I had enough money), I would buy a larger house.
if only引起的感叹句，相当于 “How I wish + 宾语从句”，意思是“但愿……;要是……就好了”
If only he could come! 他要是能来就好了!
If only we students didn’t have so much homework!要是没有这么多的作业该多好!
If only I hadn’t been so careless in the exam! 我当时没有那么粗心就好了!
if it were not for… (= were it not for…)
if it hadn’t been for… (= had it not been for…)“要不是因为有……;如果不是……”
[注意]这种结构中不能用否定结构的缩写形式，即不能用weren’t it for…)
If it hadn’t been for (= Had it not been for) the determined captain, all the passengers on board wouldn’t have been saved. 要不是船长一直坚强，船上的旅客就不会得救。
If it were not for your rich parents, you couldn’t live so easy a life. 要不是你父母有钱，你的生活不会如此安逸。
If it were not for the expense, I would go abroad now. 如果不是因为经费问题，我现在就出国了。