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Instead of using it to ask a question, can you use the question mark to express uncertainty?

Who did that?

I don't know?

I always did this, but now I am not sure if I was using it wrongly all this time.

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    Only Who did that? is a question. Commented Jan 9, 2022 at 21:40

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The basic function of question marks is to mark questions, not doubt. "I don't know." is a statement and not a question and so won't be marked with a question mark.

Sometimes question marks are used to indicate that question intonation would be used. It is possible to ask a question using only the tone of your voice.

— What's this? <- question

— It's a kind of fish. <- not a question.

— It's a fish? But it doesn't have any fins! < The first part is a question. The question is formed by rising intonation. The second part expresses doubt or incredulity. The second part isn't a question even though it implies a question "How can a fish not have fins?"

— I think you would say it's a fish. It's actually a lamprey, which is a very primitive animal. <- There is doubt in the first sentence, but it is not a question, so no question mark.

You sometimes see the question mark in brackets as a kind of editor's mark to express uncertainty:

Margaret Thatcher was the first woman, and the first redhead(?) to be Prime Minister of the UK.

The writer is expressing doubt about the "first redhead". [By the way it's not true, Churchill was a redhead, and Thatcher's colour came from a bottle]

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A question mark only ends a direct question. One must not be used to end a statement or indirect question expressing curiosity, hope, doubt, lack of knowledge, etc.

Will Mary be at the party? - correct

I wonder if Mary will be at the party? - incorrect.

I don't know? - incorrect.

You can use a question mark when a sentence is half statement and half question:

You do care, don't you?

Question marks are not used at the end of reported questions:

She wondered if it would rain that day. - correct.

She wondered if it would rain that day? - incorrect.

Question marks

The question mark

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  • I generally agree, but see my answer for an exception, which might be helpful for the person asking this question, depending on what level he or she is at.
    – cruthers
    Commented Jan 9, 2022 at 22:01
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In conversation or conversational writing, you can sometimes use a question mark at the end of a declarative statement to turn it into a question. For example:

Your friend: "I've gotta get home - my dog's gotta be hungry by now." You: "Oh, you've got a dog?"

The question in a form of a statement would be common here. Asked this way, you indicate that you just got a piece of information that makes it fairly clear that your friend has a dog, but you're asking anyway to confirm it. If you instead said, "Do you have a dog?" it would actually sound somewhat unnatural, in my opinion.

Your example doesn't fit into this category. When you say, "I don't know," you're not typically asking a question in the form of a statement. So you'd normally never put a question mark at the end of it.

In a few rare scenarios, it might nonetheless make sense to say "I don't know?" For example, if you want to express consternation that you're being asked the question in the first place, the use of the question mark (or a rising tone, if spoken) can convey something like, "I don't know... why are you asking me?" Note, however, that this could be taken with offense. The technique might also be used to express that you're not sure whether "I don't know" is an acceptable answer. There might be a few other remote applications where it would make sense to use the question mark here, but all of them together don't amount to very much.

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