would&could

Would explain:
v.aux.
1. (will的過去式,表示過去未來式)將
She said she would come.
她說過她會來的。
2. (表示意志)願,要;偏要
I told her to stop nagging, but she just wouldn't listen.
我叫她別再嘮叨,可她就是不聽。
3. (表示過去的習慣)總是,總會
He would sit silent for hours.
他常常接連好幾個小時默默地坐著。
4. (表示推測)大概,會
I think he would accept the invitation.
我想他會接受邀請的。
5. (表示喜歡,意願)願意,要,想
I would like to have a word with you.
我想同你說句話。
6. (表示虛擬,假設,條件等)要,將要;會,就會
If I were younger, I would join you in the expedition to the Antarctic.
如果我年輕些,我就會跟你們一起去南極探險。
7. (表示婉轉的請求)請
Would it be convenient for you to leave tomorrow?
明天走你方便嗎?
Would you please come tonight?
你今晚來一下好嗎?
8. (表示願望)但願,要是...多好
I wish you wouldn't smoke any more.
我希望你別再抽煙了。
9. 能,能夠
The hall would seat one thousand people.
這個大廳能坐一千人。

Could exlpain:
v.aux.
1. can的過去式
She could play the violin when she was five.
她五歲時便能拉小提琴了。
2. (用於假設語氣,表示與事實相反的設想)能,可以
I wish I could go to France.
但願我能去法國。
I would go if I could.
能去我就去。
3. (表示可能性)可能
That could be my train.
那可能是我要乘的火車。
4. (用於婉轉語氣)能,可以
Could I smoke here?
我可以在這裡抽煙嗎?
Couldn't you try it again?
你可否再試一下?

would 就是解 (表示意志)願,要;偏要 / (表示過去的習慣)總是,總會 / (表示推測)大概,會 /
(表示喜歡,意願)願意,要,想 / (表示虛擬,假設,條件等)要,將要;會,就會 / (表示婉轉的請求)請
/ (表示願望)但願,要是...多好 / 能,能夠

Could 就是解 (用於假設語氣,表示與事實相反的設想)能,可以 / (表示可能性)可能 /
(用於婉轉語氣)能,可以

其實兩個的分別可以是相反的解釋。

http://hk.dictionary.yahoo.com/search.html?q=1&s=could
http://hk.dictionary.yahoo.com/search.html?q=1&s=would

would是will 的past t，意思是將會
could是can的past t，意思是可能or 能

分別就係意思不同而已....................