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.
如果他向我提出，我可以帮助他。
If she isn't in, leave her a note.
如果她不在家，给她留一个条子。
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.
如果他向我提出了，我就帮助他。(但他没向我提出)
If she had left five minutes earlier, she would have caught the train.
如果她早五分钟动身，她就赶上火车了。(但动身晚了)
2.谈现在或将求情况，动词为：过去式would+动词原型
If I had the money, I would travel round the world
如果我有钱我将作环球旅行。(但我没有钱)
I would help you if I could.
如果能我是会帮助你的。(但不可能)
动词为be时，多用were这个形式：
If I were you, I would accept the job.
我要是你我会接受这份工作。
有时if可以省略，这时后面要用倒装语序：
Had I taken (=If I had taken) your advice, I wouldn't have made that mistake.
如果我听了你的劝告，我就不会犯那个错误了。
Were I in your position, I would stay.
我要是处在你的地位我会留下。
条件从句 主句
had+过去分词 would have +过去分词
条件从句 主句
过去式 would +动词原形
