条件句有两种，一种是真实条件句(Sentences of Real Condition)，表示一个可能实现的情况：
I'll help him if he asks me.
If she isn't in, leave her a note.
另一种是虚拟条件句(Sentences of Unreal Condition)，表示一种纯然假想的情况或实现的可能性不大的情况，这时动词需用特殊形式，称为虚拟语气，虚拟条件句有两种：
1.谈过去的情况，动词为：had+过去分词, would have+过去分词
If he had asked me, I would have helped him.
If she had left five minutes earlier, she would have caught the train.
If I had the money, I would travel round the world
I would help you if I could.
If I were you, I would accept the job.
Had I taken (=If I had taken) your advice, I wouldn't have made that mistake.
Were I in your position, I would stay.
had+过去分词 would have +过去分词
过去式 would +动词原形