如:

Could it just stay this simple？

這個是個自問自答的問句，意思是:可不可以一直這樣簡單？

could 表示請求、徵詢許可:

常用句型Could you（please）...？ 表示禮貌地請求對方做某事，意為“你（您）可以……嗎？”;Could I/we ...？ 表示希望得到對方的許可，意為“我/我們可以……嗎？”。

如：

Could I have a look at your phone？ 我可以看看你的手機嗎？

對於could引導的表示“請求、許可”的一般疑問句，其肯定回答常用“Sure. / Certainly. / Of course. / Yes， please. / OK.”等;其否定回答常用“Sorry （I’m sorry）， I （you） can’t. / No， I （you） can’t. / I’m afraid I （you） can’t.”等。

無論是肯定回答，還是否定回答，都不可用could，而要用can。

如：

—Could I have a talk with Mr. Green， please？ 我可以和格林先生談談嗎？

—Certainly. 當然可以。

—Could you please look after my cat？ 請你照看一下我的貓，好嗎？

—Sorry， I can’t. I have to go on business. 對不起，我要出差。

must和could都是情態動詞，must表示肯定推測,後面用動詞原型，它的反義詞是否定推測用can't be。could是can的過去式，也是can的委婉表達，表示能夠，後面也用動詞原型

can be和could be是時態的差別，有時候could be表語氣的委婉。

must be是肯定的推測，can't be是否定的推測，還有may be， might be等都可以表推測

1. can只有兩種時態, 即can 和could, 而be able to 有多種時態, 如:was/were able to, will/shall be able to, have/has been able to等:

I’ll be able to drive the car in a week.

Through his diligent work, the deaf-mute has been able to read and write in a sign language.

2．表示過去通過努力終於做成了某事要用be able to，而不能用can。

如：

I was able to swim to the bank after the boat turned over.

can 和be able to 都可以表示能力.can泛指一般的能力,而且只有兩種形式,即:can, could．be able to 則主要指具體做到了某件事的能力，其形式主要是靠be 發生變化，所以形式比can 多．

可以說：I can swim. I am able to swim. 但是不能說：All the people could escape from the big fire in time.

只能說：All the people were able to escape from the fire in time.

3．can能表猜測，be able to不能。can的這種用法主要用於疑問句和否定句。

Somebody is knocking at the door. Who can it be?

It can’t be our teacher who is knocking at the door.

其肯定形式是must be。如:

It must be our teacher who is knocking at the door.

must have done something 表示對現在完成或過去動作的猜測，其否定形式是can’t have done。如:

“His parents must have gone abroad.” “No, I don’t think so. They can’t have gone, for I saw them only this morning.”

要特別注意它們的反意問句形式：

He must be a good teacher，mustn’t /isn’t he？

The students must have gone to bed, haven’t they?

They must have finished their lessons yesterday, didn’t they?

could既可以用肯定形式，又可以用疑問或否定形式。如果用could表示現在“可能”，比can語氣更加委婉。如：

If you don’t spend enough time on your studies, you could fail in the final examination.

It could be our teacher who turned off the light last night.

與It must be our teacher. 相比,這句話表示的可能性更小, 因為它表示了兩種可能性, 既 “可能是”也可能“不是”，而 must be 只表示了一種猜測。

could have done 這個句型有它的特殊含意，表示“本來可能做到而未做到的事”。如：

Our class team could have won the game.

再如：

He cannot be over praised for what he has done for all of us.