表示与过去相反的情况 had＋过去分词 would/should/could/might＋have＋过去分词 If he had known your address yesterday，he would have visited you.

如果他昨天知道你的地址，他就会拜访你了。 表示与将来相反的情况，是对将来的一种假设(发生的可能性不大) should＋动词原形；were to＋动词原形；动词过去式(be动词常用were) would/should/could/might＋动词原形 1.If it should rain tomorrow，we'd stay at home.(下雨的可能性并不大)

如果明天下雨，我们就待在家里。

2.If I were to meet Mary some day，I would tell her what has happened here.

如果有一天我遇到玛丽，我就会告诉她这里发生的一切。

3.If our train arrived on time，we would have time to visit your sister.

如果我们的火车能按时到达，我们就有时间去看你妹妹。

注意：

1．虚拟条件句中有had，should，were时，可将if去掉，把had，should，were提到主语之前，即倒装结构，如：

Were they here now，they could help us.如果现在他们在这里，他们就能帮助我们。

Had I worked harder at school，I'd have got a better job.如果我在学校学习更努力的话，就会找到一份更好的工作。

Should he agree to go there，we would send him there.要是他答应去的话，我们就派他去。

2．有时条件从句表示的动作和主句表示的动作发生的时间不一致，这类句子称为错综时间条件句。此时主、从句动词的形式应根据各自所表示的时间进行调整。

If he had taken my advice then，he wouldn't be in trouble now.

如果那时他听取了我的建议，现在他就不会有麻烦了。

If she were to leave，I would have heard about it.

如果她要走，我会听说的。

3．某些介词或介词短语，如but for，without，副词或连词，如otherwise，however，or，but等也可以表达一个暗含的虚拟的条件，这种情况下要仔细阅读上下文的语境。

二、使用虚拟语气的其他情况

1．(1)在wish后的宾语从句，表示现在的愿望，谓语常用过去时。

I wish I knew his address.我要是知道他的地址就好了。