虚拟语气后的时态要提前时态么

Question

比如用一般现在时时用现在完成时
可为什么有的题目用过去时啊
时态前后是怎么排的？

Answer 1

从句的谓语动词的时态要往后推一个时态，
1．与现在的事实相反的假设的动词形式
从句：过去式（be只用were）
主句：would(should/ could / might)+动词原形
If it were not raining, we should go for a picnic.
2．与过去的事实相反的假设的动词形式
从句：had +done
主句：would(should/ could / might)+have + p.p.
She should have gone to the party if she had been invited.
3．与将来的事实相反的假设的动词形式
从句：过去式或should / were to+动词原形
主句：would(should/ could / might)+动词原形
If I were to be 20 years old next year, I should take the course of computer science.

Answer 2

在if条件句中，虚拟语气需要提前时态。
如：对现在的虚拟，用一般过去式，
对过去的虚拟, 用过去完成时，
对将来的虚拟，可以用were to/should do/一般过去时，

而如果是在名词性从句中，如主语从句、宾语从句等中出现，只需要用（should）+动词原形 就可以了。

Answer 3

if 分为3种情况：从句if 主句
现在时 would+动词原形
过去时 would+have+动词原形
将来时 were to
或should do
或would+动词原形
But for要看题中的时间状语 常与过去事实相反
If it had not been for 与过去事实相反
我就记着这些 其他的有点忘了 想起来给你补充