title: 英语中，主句，从句的时态，有规定吗 
answer: 主句从句各用时态
  答：在条件句中的应用的规律是：
  
  虚拟语气在条件句中的应用
  
  包含条件从句的句子称为条件句。条件句有两类，一类是真实条件句（Sentences of Real ondition ），一类是虚拟条件句（Sentences of Unreal Condition ）如果假设的情况是有可能发生的，就是真实条件句：
  
  If he has time, he will come. 如果他有时间他是会来的。
  
  如果是纯然假设的情况或是发生的可能性不大，则是虚拟条件句：
  
  If I were you, I would reconsider their proposal.
  
  如果我是你，我就重新考虑他们的建议。
  
  1） 表示现在及将来情况的虚拟条件句（表示纯然假设或实现的可能性不大）：
  
  谓语主要形式如下：
  
  
  
  从 句
  主 句
  
  过 去 式
  Would + 动词原形
  
  
  
  
  Be的 过去式用were 。
  
  If I had time, I would certainly go with them.
  
  要是有时间我一定去。（事实上我没时间。）
  
  How nice it would be if he could stay a bit longer!
  
  要是他能多待些时候该多好啊！（可惜他不能多待。）
  
  2）表示过去情况的虚拟条件句，谓语主要
  
  
  
  从 句
  主 句
  
  had + 过去分词
  Would have + 动词原形
  
  
  
  
  If he had left a little earlier, he would have caught the train.
  
  他要是早点动身就赶上火车了。（事实上他没有早动身。）
  
  If she hadn’t taken your advice, she would have made a bad mistake.
  
  她要是不听了你的劝告就犯大错了。（事实上她听了你的劝告，所以你没犯大错。）
  
  2） 在以上两类虚拟条件句中，主句的谓语动词除了用would + 动词原形外，
  
  有时还可以用
  
  might 或could +动词原形：
  
  If she were here, she might be able to help.
  
  要是她在这里她可能会帮忙。
  
  If he had been more careful, he could have done better.
  
  你要是细心点儿，是可以做得更好的。
  
  4）主句第一人称后有时也可以用should (但美国人多用would).
  
  If they were here with us, we should / would feel much better.
  
  如果他们和我们在一起，我们会感到好得多。
  
  5）在表示将来情况的条件从句中，有时可用 “were to / should + 动词原形” 构成
  
  谓语。
  
  If I were to / should do it, I would do it in a different way.
  
  要是我来做这件事，我会是另一种做法。
  
  6）有时可把 if 省略掉，但主谓结构要倒装。
  
  Had they time ( = if they had time), they would certainly come and help us.
  
  Should there be a flood ( = if there should be a flood ), what should we do?
  
  