②If it rained tomorrow, our picnic would be put off. (Illustration：The weather has been very good these days.)
条件状语从句的谓语动词用过去时,表示说话人认为下雨的可能性不大。
③If it should rain tomorrow, our picnic would be put off. (万一明天下雨,我们的野餐就推迟)。
条件状语从句的谓语动词用should+动词原形,说明下雨的可能性很小或没有可能,如果下雨,那将是出乎意料的。
④If it were to rain tomorrow, our picnic would be put off.这句话与上一句意思几乎一模一样。
4、混合时间的虚拟语气
当条件状语从句表示的行为和主句表示的行为所发生的时间不一致时,主句和从句的谓语动词的形式应分别根据各自所表示的时间加以调整。这与以上三种情况不同,虚拟语气的谓语动词形式没有遵循以上规律。
例如：If I had received the passport yesterday, I would start today. (从句与过去事实相反,主句与现在事实相反。)
If China had not been liberated, the working people would still be leading a miserable life. 5、倒装句中的虚拟语气
在书面语中,如果条件状语从句的谓语中有were, had 或should, 就可以省略if, 并将were, had , could或should 放到句首,谓语主语之前,用 “Were/Had/Should/could+ 主
语”的形式。这种虚拟语气在意义上与带if 的条件状语从句相同。如果从句没有were, had, could或should,就不能省略if。而将条件从句的主语置于were, had, should, could之后。
例如：If you had been here earlier, you would have seen him.→Had you been here earlier, you would have seen him.
在虚拟条件状语从句中,省略了if的倒装形式的句首不能用动词的缩略形式。
例如：Were it not for the expense, I would go to Italy.(√); Weren’t it for the expense, I would go to Italy.(×); 6、宾语从句中的虚拟语气
在表示命令、建议、要求等一类动词后面的宾语从句中,用虚拟语气。如beg, move(提议), arrange, order, suggest(建议), propose, require, desire, demand, request, insist(坚持要某人做某事时), command, insist +should do。Should可以省略,但不可用would等来替代,引导宾语从句的that不能省略。主句所使用的动词时态不限。
例如：The doctor ordered that she (should) stay in bed for a few days./The commander ordered that the bridge (should) be bombed
注： suggest表示“暗示”时,宾语从句不用虚拟语气。Insist表示“坚持认为”,后面强调的是事实时,宾语从句也不用虚拟语气。
例如：The neighbor suggested that his wife was having an affair.
(错) You pale face suggests that you(should)be ill./(对) Your pale face
suggests that you are ill.
(错) I insisted that you(should)be wrong./(对) I insisted that you were
wrong.
7、表语从句,同位语从句中的虚拟语气
suggestion, proposal, request, idea, plan, order, advice等名词后面的表语从句、同位语从句中要用虚拟语气,即(should)+动词原形。should 可以省略,从句的引导词that 不能省略。
例如：I make a proposal that we(should)hold a meeting next week.
My idea is that we(should)get more people to attend the conference.
8、主语从句中的虚拟语气
It is demanded / necessary / a pity + that…等结构的主语从句,谓语动词用should 加动词原形,should 可省略。 It is 可用的词有三类 suggested, ordered, required, proposed, demanded, requested, insisted等 important, necessary, natural, imperative, strange, essential, vital, desirable, advisable, better等 a pity, a shame, no wonder等 that (should)do 9、比较if only与only if ：only if表示\只有\；if only则表示\如果……就好了\。If only多用于虚拟语气,但也用于陈述语气。
例如：I wake up only if the alarm clock rings. /If only the alarm clock had rung. 10、几个常用虚拟语气的特殊句型
⑴It is (high) time (that) “(现在)该…”句型
It is(high)time that后面的从句谓语动词要用过去式(be的过去式用were)或用should加动词原形,但should不可省略。
例如：It is high time that the children should go to bed.
⑵I would rather (that)…“我宁可…”句型：谓语动词用过去式(be的过去式用were)
例如：I’d rather I were in the rain now. ⑶as if/as though,或even if/even though引导的从句
只看从句动作发生的时间,与主句的时态没有关系。如果从句中表示的动作发生在过去,则谓语动词用had+过去分词形式;如果指的时现在的状况,则用过去式(be用were);指将来则用would(should, could)+动词原形。
例如：He speaks English so fluently as if he had studied English in the U.S.
Even if he were here, he could not solve the problem.
⑷Suppose(that)祈使句
从句的谓语动词变化与as if后面的动词变化规律相同,用过去式、过去完成式、或过去将来式。意思是“假如…”,但suppose 是动词,不是连词。 ⑸for fear that或lest引导的从句
表示“怕…”“万一…”等意思,从句的谓语动词用should+动词原形,should 可以省略。 例如：The officer forbade the use of artillery lest the city’s industry be damaged.
Lest the wall should collapse, they evacuated from the building.
She closed the windows for fear that she (should) catch cold.
⑹习惯用语,如：May you be happy! 11、wish的用法
1)wish后面的宾语从句的谓语动词应使用虚拟语气, 表示很难实现或不能实现的愿望,可以翻译为“可惜…；….就好了； 悔不该…； 但愿…。”其宾语从句的动词形式为：
主句谓语 时态 现在时,表示与wish同时发生 wish 过去时,表示在wish之前发生的动作 从句谓语 谓语动词的形式 动词用过去时,be动词用were 动词用had done,be用had been 动词用would do; should do,be用would 将来时,表示在wish之后发生的动作 be ; should be 例如：I wish it would rain tomorrow. 我希望明天下雨就好了。 2) wish to do；wish sb / sth to do。
例如：I wish the manager to be informed at once.
注：wish 与hope接宾语从句的区别在于：hope表示一般可以实现的希望,宾语从句用陈述语气。Wish表示很难或不大可能实现的愿望,宾语从句用虚拟语气。 12、其它
1)用介词短语without…/ But for/except…代替条件状语从句(可以改为条件状语从句)。
例如：Without proper means of control, a nuclear reactor would explode. 如果没有恰当的控制手段,核反应堆就会爆炸。