當我哋想用英文表達「應該、可以、想要」去做某件事，但冇去做，應該點樣講呢？最常用嘅就係could have, would have, should have啦，但係唔少人其實唔係好清楚佢哋係用法上有咩分別，係考試嘅時候會唔小心用錯！我哋會詳細說明佢哋嘅用法同分別，一齊黎睇下啦！
1. Could have
「Could have + 過去分詞」用來表達原本可能可以做，或過去有能力做，但是卻沒有去做。
I could have gone directly to college, but I decided to travel for a year.
「Could have + 過去分詞」還有另外一個用法，就是當你表示對某個人說的話強烈贊同的時候，可以說「I couldn’t have said it better myself」。
「Couldn’t have + 過去分詞」，則是用來表達，某件事是沒可能達成的。
She could not have been on that flight because I just saw her at work.
2. Would have
「would have + 過去分詞」用來表達某件事你很想做，但卻沒去做，就用「I would have A, but I had to B」。
I would have called, but there was no phone service.
Would have也可以構成過去虛擬條件（past unreal conditional）的結果從句，比如：
If I had known they were vegetarians, I would have made a salad.
3. Should have
「should have + 過去分詞」用來表達某件事是你應該去做的，但卻沒做。
I’m sorry that I’m late for work. I should have woken up earlier.
should have有時候會加上by now，用來表達某件事應該如何發生，但卻沒發生。
The plane should have arrived by now.