If you had come to class, you would have passed the exam. 如果你有来上课，你就会通过考试。 (你没来上课, 也没有通过考试)
If he hadn’t had his helmet on, he might have died. 如果他没有戴安全帽，他可能会死掉。 (因为他有戴安全帽,所以没有死。)
if + 过去完成式表达一个在过去想像或假设的情况。可以在主要从句用 would have, could have 或 might have + 过去分词来表达某个想像情境的结果。
If you had talked to me when you saw me, you would have met my friend. 如果你在看到我的时候和我说话，你就会遇到我的朋友。
If she had taken her wallet, she could have bought what she wanted. 如果她带了钱包，她就可以买到她想要的东西。
If the volunteers had found the missing person, they might have saved his life. 如果志愿者找到了失踪者，他们可能会挽救他的生命。
如同所有的条件型，当 if 从句出现在句子前面，后面要加逗号，但是如果 if 从句出现在句子后面，则不需要加上逗号。
If I hadn’t married early, I wouldn’t have been miserable.
I wouldn’t have been miserable if I hadn’t married early.
在条件句子中也可以用 unless 来代表 if… (not)
I wouldn’t have stayed in the villa unless it had rained. – I wouldn’t have stayed in the villa if it hadn’t rained. 除非下雨，否则我不会呆在别墅里。
第三条件句 – 练习题