Would, should, could, might并非仅仅作为will, shall, can, may的过去式而存在。实际上它们可能表达完全不同的概念。He can do it 意为“他肯定可以做”（一个事实），而He could do it意为我估计“他可以做”（一个估测），尽管他还没有做。
当表达“纯粹估测”的概念时，我们用would, should, could, might。学会它们的准确使用是讲好英语的首要条件之一。仔细推敲下列例句：
What will you do now? (What are you going to do?)----一个关于事实的问题
What would you do (if you were in my place)?----一个估测
Can you do it? (Are you able to do it?)----一个事实
Could you do it? (If I should ask you)?----一个估测
Suppose you put water over the fire, what would happen? Would the fire get wet?
Suppose you put oil into the fire, what would happen? You might burn down the whole house, might you not?
I should like to see you try. (I suppose I like.)
I should think he feel rather ashamed of it.
You should help him if you could. And I belive you could if you would. (You will not, that's why you say you can't.)
