Will 是未來式：I will never cheat on you. Would 是過去的未來式： He said that he would never cheat on you. 然而，would 還帶有假設語氣的用法：if I were him, I would never cheat on you. 除了 if，用在表達想法的句子裡也應該使用假設語氣：I think the meal would be enough for ten people.
Can 是能夠的意思： I can run for miles. Could 則是過去式：When I was younger, I could run for miles. 然而，could 也帶有假設語氣的用法：If I were young, I could run for miles. 除了 if，用在表達想法的句子裡也應使用假設語氣： I think the meal could feed ten people.
could 較 would 多了一個「可能性」的意思： We could have as many as ten people for dinner tonight. 意思是「我們可能會有十個客人」，若這邊使用 can： We can have as many as ten people for dinner tonight. 意思會變成「我們有能力招待十個客人」
Would 和 Could 都有禮貌的用法：Could I borrow your car? Would you mind if I borrow your car?