对现在的推测

She could be at home.

It could rain later today.

对将来的推测

They could be arriving soon.

We could go on vacation next month.

对过去的推测

He could have finished the work by now.

She couldn't have left so early.

礼貌地提出请求

Could you please help me with this？

Could I ask for a favor？

表示不确定性

He could be the one who stole the wallet.

I'm not sure, but she could be coming to the party.

这些例句展示了"could"在不同语境下用于表示推测的用法，既可以在肯定句中使用，也可以在否定句和疑问句中使用，并且语气较为委婉和不确定。希望这些例句对你有所帮助。