与过去事实相反，从句谓语动词：had done
将来可能性不大，从句谓语动词：would /could / should / might +do
六、目的状语从句
1、在for fear that, in case, lest引导的从句中，若用虚拟语气时，从句谓语为：（should ）+ do。
She examined the door again for fear that a thief (should) come in. 她又把门检查了一遍，以防盗贼的进入。
He started out earlier lest he (should) be late. 他早早的就出发了以防迟到。
2、so that, in order that所引导目的状语从句中，
从句中谓语为：can / could / may / might / will / would / should + do
He goes closer to the speaker so that he can hear him clearer. 他走近说话的人以便能听得更清楚。
He read the letter carefully in order that he should not miss a word. 他把信读得很仔细以便不漏掉一个字。
七、虚拟语气还可用在定语从句中，表示：“早该做某事了”时，谓语动词须用虚拟语气，从句虚拟语气结构：
It is (high / about) time that + 主语 + 动词的过去式 should + do（should不可省，且优先使用动词过去式）
It is time for sb. to do sth.
It is time that I went to pick up my daughter at school. 我该去学校接我的女儿了。
It is high time you should go to work. 你早该上班了。 It is （the first / second / third…）time that 从句谓语动词用have done
It was（the first / second / third…）time that 从句谓语动词用had done
八、固定搭配
1. Were it not for…“要不是……”与现在事实相反： Were it not for your help, I would never be a good boss. 2. Had it not been for…“当时要不是”与过去事实相反： Had it not been for you, I would have lost my way. 3. What if “要是…该怎么办？”if从句谓语用should do:
What if it should rain heavily tomorrow?
4. Would you mind + if “你介不介意，如果……”从句谓语多用did:
Would you mind if I opened the window?
辨别would，could，should，might Would 必然的结果 Could 能够 Might 可能的结果
Should 在主句中，它的主语只能是第一人称I或we