how canI write the sentence in English?

What's a key for? Or
What do you use a key for?

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Both lines are appropriate.
Why?
Because the answer to this question is: To unlock the door.
Out of the two, I think the first one comes out better.
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You don't have to translate Chinese to other foreign language word for word. You'd rather find a way to describe it. Otherwise, it makes it awkward and funny (funny here actually means creating a low-grade image upon yourself). What it means in Chinese sometimes does not fit in or carry the actual meaning when translated into English.

To get the real hang of it, you have to live with people speaking proper English for a long while. I know it's hard 'cause I've been with them for almost 40 years and still don't know enough.

Sorry .

What are the things that have locks which need keys to open?

"What are the things that need a key to open?"

What things need to be opened by a key?

你果句what do we need a key to open?
= 你需要鎖匙去開什麼? 句法類似 What do you need me to do?

同 '有什麼是用鑰匙去開的?' 有小小分別.

What things enable us to open with a key?

What can be opend by a key?
or
What things are opend by a key?