虚拟语气：

(1) 出现but for（要不是）, but that（若不，后接一从句）, without , otherwise, if only ,as if (though)，it's (high, about) time that…, I would (had) rather…, 时，要用虚拟语气。

(2) 在It's necessary (best, important, imperative, natural, desired, requested, ordered, decided, a pity, a shame, incredible, no wonder, essential , proposed , required, suggested , essential, urgent recommendable, desirable/,advisable, impossible, strange )that…should + 动原 结构中，that引导的真正主语从句中动词应用虚拟式。

(3) 与将来时间相反。从句的表达法：1、动词的过去式；2、were +不定式；3、should +动词原形；主句:would (should, could ,might)+动词原形

(4) 当条件从句表示的动作和主句表示的动作发生的时间不一致时，动词的形式要根据时间调整。If he had received six more votes, he would be our chairman now.

(5) suggestion, motion, proposal, order, recommendation , plan, idea等后面的表语从句和同位从句, 用虚拟语气。

(6) 在ask, require , order, demand, suggest ,propose ,command, insist，request, desire recommend, propose, advised, urge等动词后的宾语从句中，动词用虚拟式，表示命令、建议等。insist 如作"坚持要求"，用虚拟，如作"坚持认为"，则不用虚拟式。

(7) expect, believe, think, suspect 等动词的否定式或疑问式后的宾主从句表示惊奇和怀疑、不满等意义时常用虚拟语气。

I never expected that the problem (should)be so complicated.

I'm sorry , we were surprised, they were disappointed +should +动词原形