1. 表示与现在事实相反，条件从句谓语动词用过去式(如果动词是be，多用were)，主句谓语动词用“would (should, could, might) + 动词原形”。
If they were here, they would help you.
2. 表示与过去事实相反，条件从句谓语动词用“had + 过去分词”，主句谓语动词用“would (should, could, might) + have + 过去分词”。
If she had worked harder, she would have succeeded.
3. 表示与将来事实相反，条件从句谓语动词用过去式或者“should + 动词原形”，或者“were to + 动词原形”，主句谓语动词用“would(should, could, might) + 动词原形”。
If you succeeded / should succeed / were to succeed, everything would be all right.
If you had asked him yesterday, you would know what to do now.
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。
Were they here now, they could help us. / If they were here now, they could help us.
If I were you, I would go to look for him. 如果我是你，就会去找他。
Weren’t it for the expense, I would go to Italy. (错)
Were it not for the expense, I would go to Italy. (对)
二、用with, without, but for(要不是), if only(要是), under, otherwise, or引导的短语，以及分词短语、独立主格结构、不定式短语表示的虚拟条件，若能用if条件句改写，谓语动词要按主句在现在、过去和将来的虚拟语气形式分别处理。
Everything taken into consideration, they would have raised their output quickly. (独立主格结构)
三、在as if, as though, even if, even though引导的从句中，谓语动词也按条件从句在现在、过去和将来的虚拟语气形式分别处理。
Even if he were here, he could not solve the problem.
I wish it were spring all the year round.
2. 在advise, command, demand, order, insist(坚持), propose, suggest(建议), recommend, request等后面的宾语从句中用“(should) + 动词原形”。
We suggest that we (should) have a meeting.
在a pity, a shame, no wonder, strange, sorry, necessary, natural, impossible, important后面that引导的主语从句中用“(should) + 动词原形”表示惊奇、不相信、惋惜、理应如此等。
It is necessary / important / natural / strange that we should clean the room every day.
在名词advice， demand， plan， idea， order， request， proposal， suggestion后面的同位语从句或表语从句中，谓语动词用“(should) + 动词原形”的虚拟语气形式。
We all agreed to his suggestion that we (should) go to Beijing for sightseeing.
常用于It is time后面的that从句中，定语从句中谓语动词用过去式(be用were)，或“should + 动词原形”(should不能省)来表示，意为“(现在)该……”
It is (high) time we left / should leave.
It is high time we were going.
I would rather we were at home now. (现在)
I would rather nothing had happened. (过去)
I would rather you came tomorrow.