摘要:A if从句中的动词用过去完成时，主句中的动词用条件完成时。这种句子所指的时间是过去；所提出的条件是不可能实现的，因为if从句中的动作在过去并未发生：If I had known that you were coming I would have met you[阅读全文]

摘要:A if从句中的动词用一般过去时，主句中的动词用条件时态：If I had a map I would lend it to you．我要有地图就借给你。（但我没有地图。句子的含义是指现在。）If someone tried to blackmail me I would tell the[阅读全文]

摘要:A if从句中的动词是一般现在时，主句的动词是一般将来时,主句和从句的先后次序无关紧要：If he runs he’ll get there in time．如果他跑步去就会及时赶到那里。The cat will scratch you if you pull her tail．如果[阅读全文]