一 、通过动词形态的变化
与现在相反：从句一般过去时–主句would/should+动词原形
与过去相反：从句过去完成时–主句would/should + have+ 过去分词
与将来相反：从句were + 不定式–主句would/should+动词原形
二、should+动词原形
1、It is +important/ necessary/ possible/ ridiculous/ urgent/ vital/ proposed等形容词+ that从句, e.g.:
It’s necessary that we (should) set out at once.
2、表示”提议、建议、请求、命令”(suggest/ insist/ command/demand/intend/order/ propose)等意思的动词所带的从句, e.g.
I suggest that he (should) go camping tomorrow.
3、在lest, for fear that, in case等词引导的从句中，谓语部分用should+动词原形。e.g.:
Grown people should never say or do anything wrong before children, lest they (should) set them a bad example.
三、谓语动词提前一个时态(在原来时态的基础上再”过去”一下)
1.wish后的宾语从句，e.g.:
I wish I had been to the concert last night.
I would (had) rather后的句子
I’d rather he hadn’t done anything like that.
四、谓语动词用一般过去时
It’s (high, about) time that…, e.g.:
It’s high time that he stopped smoking.
五、由as if，as though引导的从句中，表示对现在的怀疑：用一般过去时
对过去的怀疑：用过去完成时 e.g.:
She looks as if she knew all about it.
if only引导的感叹句中，表示与现在相反：用一般过去时
表示与过去相反：用过去完成时 e.g.:
If only he knew our telephone number!