一般过去时 would / should + 动词原形

were+ 不定式 would/should + 动词原形

should+ 动词原形 would/should + 动词原形

If you succeeded, everything would be all right.

If you should succeed, everything would be all right.

If you were to succeed, everything would be all right.

混合条件句

主句与从句的动作发生在不同的时间，这时主，从句谓语动词的虚拟语气形式因时间不同而不同，这叫做混合条件句。这时找准时间状语是关键。

If you had asked him yesterday, you would know what to do now. （从句与过去事实相反，主句与现在事实相反。）

If it had rained last night (过去), it would be very cold today (现在). 虚拟条件句的倒装

虚拟条件句的从句部分如果含有were, should, 或had, 可将if省略，再把were, should或had 移到从句句首，实行倒装。