如题所述
1、表示与现在事实相反的，if从句用过去式，主句用would （或could、should、might）加原形动词。
2、与将来事实可能相反的，if 从句用should （或were to ）加动词原形，主句用would加动词原形。
在笔语中，条件从句有时可以不用连词小而把were，had或should移至主语之前。但如从句没有were，had或should，则一般不能这样做
如：Should he come，tell him to ring me up.
他要是来了，让他给我打个电话。
If I were you, I would helphim. （对现在的虚拟，假如我是你）
If it were to rain tomorrow, Iwould not go hiking. （对将来的虚拟）
扩展资料
其他用法：
有时条件从句中的动作和结果主句中的动作所发生的时间不一致
1、这时，动词的形式应根据它所表示的时间加以调整。
如：If he had followed the doctor's advice，he Would be quite all right now.
他当时若是听医生的话，现在就会痊愈了。
从句说明过去，主句说明现在
If I were you，I Would have gone to the theatre.
假如我是你，我会去看话剧了。
从句的时间包括现在，主句说明过去
If you hadn't helped me，I couldn't have finished it on time and most likely I would be still working now.
假若你没有帮助我，我不可能按时完成工作，很可能现在还在干哩。
2、用直陈语气表示条件和结果
在现代英语里，多用直陈语气表示条件和结果。直陈语气的条件和结果所表示的往往是事实。如不是事实，其可能性也较虚拟语气要大。
如：If it rains tomorrow，we shall not go out.
假如明天下雨的话，我们就不出去。
The dog won't attack you if you sit still.
如果你安静地坐着，狗是不会咬你的。
与现在事实相反：从句动词用：did/were; 主句动词用：would do/ be
与将来事实相反：从句动词用：should(were to) do /be 主句动词用：would do/ be
用在以if引导的非真实条件状语从句中，从句和主句皆须用虚拟语气。从句和主句动词的搭配形式，通常为：
与现在事实相反：从句动词用：did/were; 主句动词用：would do/ be
与过去事实相反：从句动词用：had done/been 主句动词用：would have done/ been
与将来事实相反：从句动词用：should(were to) do /be 主句动词用：would do/ be
三种基本形态（这是基础，建议牢记下面的例句，争取能脱口而出）
与现在事实相反：If I had enough money, I would buy a book.
与过去事实相反：If I had had enough money, I would have bought a book.
与将来事实相反：If I were to have enough money, I would buy a book.
If I should have enough money, I would buy a book.本回答被提问者采纳
