我是舉例子兼開玩笑

would 是比較肯定的語氣
知道你"能"做到也"要"你去做到
比較像是下命令一樣
例子: "Would you plesae finish your dinner first?" , "Would you please carry my luggage upstairs?" , "Would you please finish this report tonight and bring it to my office tomorrow?"


could 是比較不確定也不肯定的語氣
如果能的話可以請你怎樣怎樣.....
例子: "Could you bring me a coffee on your way back?" "Could you please help my math?"


你讀台灣的文法書會告訴你兩個都是禮貌用法.......
不是這樣的.....

低姿態請求別人的話會用 "Could you"
用"Would you" 的話會有令人感覺壓迫的意思