Could 可用來表示可能性、過去的能力、請求或建議。Could 係 can 在條件句中的用字，經常用於條件句。
例句：
- I could attend your wedding ceremony. (我可能參加你的婚禮) － 可能性
- Bill could swim the butterfly stroke by the age of 10. (比爾 10 歲之前會游蝶式) － 過去的能力
- Could I borrow some money from you to eat out with my girlfriend? (我可以向你借一些錢帶我女朋友外出用餐嗎？) － 請求
- You could go out with Mary instead. (你可以改跟瑪麗約會) － 建議
- If you went to Egypt, you could learn Arabic. (如果你去埃及，你就可以學阿拉伯語) － 現在非事實條件句 (與現在事實相反的條件句)
Could 的現在、過去和未來時態用法
大多數語氣助動詞的過去和未來時態都相當不規則。下表為 could 在不同上下文中的用法說明。
| Could 的用法 | 肯定句 | 你也可以用 |
| 可能性 | 現在：Tony could be the one who robbed the bank. (東尼可能是搶銀行的人) 過去：Tony could have been the one who robbed the bank. (東尼可能是搶銀行的人) 未來：Tony could go to jail for robbing the bank. (東尼可能因搶銀行而入獄) | might, may |
| Can 在條件句中的用字 | 現在：If I had more money, I could travel around the world. (如果我有更多的錢，我就可以去環遊世界) 過去：If I had had more money, I could have traveled around the world. (如果我有更多的錢，我就可以去環遊世界) 未來：If I had more money next year, I could travel around the world. (如果明年我有更多的錢，我就可以去環遊世界) | |
| 建議 | 現在：沒有現在式。 過去：You could have gone on a day trip to Hualien. (你們可以去花蓮一日遊) 未來：You could go on a day trip to Hualien. (你們可以去花蓮一日遊) | |
| 過去的能力 | I could run ten kilometers in my early twenties. (我二十出頭時可以跑一萬公尺) I could speak Korean when I was a child. (小時候我會說韓文) Could 在表示短暫或一時的能力時不可用於肯定句。 I could jump over a high wall yesterday. (昨天我能跳過高牆) (誤) | be able to |
| 禮貌性請求 | Could I have a cup of coffee? (可以給我一杯咖啡嗎？) Could you give me a discount if I buy a whole case of beer? (如果我買一整箱啤酒，你能給我打個折嗎？) 請求通常指不久的將來。 | can, may, might |
| Could 的用法 | 否定句 | 你也可以用 |
| 可能性 | 現在：Tony couldn't be the one who robbed the bank. (東尼不可能是搶銀行的人) 過去：Tony couldn't have been the one who robbed the bank. (東尼不可能是搶銀行的人) 未來：Tony couldn't possibly go to jail for robbing the bank. (東尼可能不會因搶銀行而入獄) | might, may |
| Can 在條件句中的用字 | 現在：Even if I had more money, I couldn't travel around the world. (即使我有更多的錢，我也不能去環遊世界) 過去：Even if I had had more money, I couldn't have traveled around the world. (即使我有更多的錢，我也不能去環遊世界) 未來：Even if I had more money next year, I couldn't travel around the world. (即使明年我有更多的錢，我也不能去環遊世界) | |
| 建議 | 沒有否定型態。 | |
| 過去的能力 | I couldn't run more than a kilometer in my early twenties. (我二十出頭時不能跑超過一千公尺) I couldn't speak Korean when I was a child. (小時候我不會說韓文) Could 在表示短暫或一時的能力時可以用於否定句。 I couldn't jump over a high wall yesterday. (昨天我不能跳過高牆) (正) | be able to |
| 禮貌性請求 | Couldn't Mary attend my birthday party? (瑪麗不能來參加我的生日派對嗎？) Couldn't you help me with my homework? (你不能教我做功課嗎？) 請求通常指不久的將來。 | can, may, might |
注意：“Could not” 和 “Might not” 的意思並不相同。Could not 意為「不可能」，而 Might not 意為「可能不；也許不」。
例句：
- Cindy could not know how to solve the problem. (辛蒂不可能知道這問題的解決辦法)
- Cindy might not know how to solve the problem. (辛蒂可能不知道這問題的解決辦法)