would,could,should,might,这4个词可以互换吗?各自什么意思？请区别性的造句

在现在时态句里，用情态动词(Modal Verb)的过去时态
(could,might,should,would)表示说话人的谦虚、客气、有礼、委婉等：基本可互换，但might很少用
1. Would you be kind enough to show me the way to the post office.
(情态动词 would,表示客气有礼)
2. It would be better for you not to stay up too late.(表示委婉)

http://cache.baidu.com/c?word=would%2Ccould%2Cshould%2Cmight&url=http%3A//flc%2Ezjgsu%2Eedu%2Ecn/englishonline/ksfd/gcss/Grammar/subjunctive%2Dmood%2Easp&b=14&a=1&user=baidu

拟语气是英文中一特殊的语言现象，主要用于表达与事实相反的陈述，常表达强烈愿望、遗憾、感慨、后悔、责备、规劝等语义，可大致分为三类

一、对现在事实的虚拟
基本形式：If + were /did等过去式…, …would /could /should /might + do
例如：If I were a bird, I would fly to the moon.
(事实上，I'm not a bird, so I will not fly to the moon.)
If she knew who you are, she would go