could 的用法

1. 作为 can 的过去式，表示能力， 能做某事。
Lily could play the piano by the time she was only six. 莉莉六岁时就能弹钢琴了。
They couldn't afford to buy a new car, so they chose a secondhand one.他们买不起新车，只能选一辆二手的。
The two children were having a competition to see who could run faster. 两个孩子在较量谁跑得快。

2. 表示提议，请求，可以或不可以。语气上比can 委婉，客气。
Could you please open the window? 可以请您开一下窗吗？
Could I have a glass of water? 我可以要一杯水吗？
You could send an email to him right now. 你可以立即写电子邮件给他。
3. (用于否定句及疑问句中)表示惊异， 不相信等。
I couldn't believe it! 我怎么能相信！
How could you be so cruel! 你怎么能如此残忍！
Could this be true? 这能是真的吗？

4. could have done sth 的用法
1）表示“那时(不)可能”。
Don't worry --- Tom could have just left the phone at school. 别担心，汤姆可能只是把电话落在学校了。
We could not have heard the bell because of the noise from the river. 由于河上传来的噪音，我们不可能听见铃声

2) 表示“本来可以” 或　“差点就”。
With a little care you could have avoided that mistake. 你要是小心一点，本来是可以避免那个错误的。
I could have lent you the money, why didn't you ask me? 我本可以借你这些钱的，为什么不问我呢？
This accident could have been prevented. 这场意外本来可以避免的。

3) 用于指出婉转的批评。
You could have been more considerate. 你本可以考虑得更周到些。
You could have informed me earlier. 你本可以早一点通知我。
You could have at least bought me a card. 你本可以至少送我张卡片。

5．用于虚拟语气【高中以上】。
If I had any money with me, I could lend you some. 如果我带钱了，我就会借给你些。
You could have passed this exam if you'd studied harder. 如果你学习再努力一点的话你这次就通过考试了。