could 用法

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一、can的过去式：用来表示过去的能力、可能性或允许，否定式是：couldn't 或 could not。

I could run faster then.

那时我能跑得更快.

She said that she couldn't come.

她说她来不了啦。

I couldn't hear what they were saying.

我听不见他们在说些什么。

Sorry, I couldn't get any more.

对不起，我无法弄到更多了。

It could be no better at that time.

那时好得不能再好了。

Only men could go to the club in those days.
在当时，只有男人可以去俱乐部。

Don't worry─they could have just forgotten to call.

别担心，他们可能只是忘了打电话。

二、委婉的提出问题或陈述看法，这时，could和can没有时间上的差别。可以用can，只是用could更客气，谦虚和委婉一些。

Could you please do it for me ?

你能帮我做这事吗？

Could you babysit for us on Friday?

你星期五帮我们看一下孩子好吗？

I could be wrong. Could you come over here?
我也许错了。你能到这边来吗?

It could be about 4:30 p.m.

可能是下午4:30左右。

三、表建议

We could write a letter to the director.

我们不妨给主管写封信。

You could always try his home number.

你总可以试着给他往家里打电话呀。

四、Could用于条件句和假设句的主句中 

If he worked hard last week, he could finish the work.---->这是真实条件句，其主句的could指将来，语气更加委婉一些。

If he had worked hard, he could have finished the work.---->这是假设句

实际上他过去并没有在努力的工作， 所以他未能完成那项工作。

五、could have + 过去分词：表示现在对过去事物的判断，可译为本来能够或者本来可以，暗指当时未实现的动作。

They could have let me know they were going to be late!

他们要晚来，也该早告诉我一声呀！

Anyone could have drugged that wine.

指不定谁在那酒里下了药。

You could have been killed!

你差点儿就没命了！

He could have taken a spare key.

他本该带上一把备用钥匙的。