1.方式状语从句
由as if 或as though引导的状语从句可以用陈述语气表示可能符合事实的情况，也可以用虚拟语气表示与事实不符或与事实相反的情况。
a. 表示与现在事实不相符，用一般过去时
The teacher treats the pupil as if he were her own child. 这位老师对待这位学生就像自己的亲生孩子一样。（这位学生并不是她的亲生孩子）
b. 表示与过去事实不相符，用过去完成时
I felt as though we had known each other for years. 我感觉我们好像认识多年了。（其实并没有认识多年）
c. 表示与将来事实不相符，用wouldmightcould
It looks as if it might rain. 天看上去好像要下雨。（不大可能下雨）
对比：
He looks as if he is young. 看样子他好像很年轻。（他就是年轻）
He looks as if he were young. 看样子他好像很年轻。（实际上他不年轻）
2.目的状语从句
a. 由in case, lest, for fear that引导的目的状语从句，动词用“should + 动词原形”结构，表示忧虑或目的。
Please remind me of it again tomorrow in case I (should) forget. 请你明天再提醒我这件事，以免我忘记。
She emphasized it again and again, lest he (should) forget. 她一再强调这一点免得他忘记。
We had a meeting and talked the matter over face to face for fear that there
should be any misunderstanding.
我们开了一个会，面对面谈了这件事，以免发生任何误会.
b. 在in order that和so that引导的目的状语从句中，往往用can(could)或may(might)。
I shall write down your telephone number so that I may not forget. 我要把你的电话号码记下来，以免忘记。They worked harder than usual in order that they could finish the work ahead of time. 为了能提前完成工作，他们比往常更加努力。We will tell you the truth so that you can judge for yourself. 我把真实情况告诉你，使你能自己作出判断。
3.让步状语从句
让步状语从句指事实时，从句谓语动词用陈述语气。若从句内容表示现在和将来的假设情况，从句谓语动词用虚拟语气。
Though he (should) fail, there would still be hope. 即使他失败了，仍有一线希望。
I should say the same thing even if he were here. 即使他在这里，我还是要这么说。
Whether he (should) succeed or fail, we shall have to do our part. 不管他成功还是失败，我们还是要做好自己的事。
However hard it might rain rains, we shall go there together. 不管雨下得多么大，今晚我们还是要去那里。
No matter what his social position (might) be, a man is equal in the eye of the law. 一个人不论其社会地位如何，在法律面前都是平等的。