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I don't know what the problem is.
I don't know which one correct is.

Some people told me the first sentence is correct and the second sentence is incorrect.
My question is:
Why using the word 'is' (at the end of the sentense) can be correct in the first sentence, but not in the second sentence?

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Correct is an adjective, and problem is a noun.

The second sentence is understandable, but it's the way Yoda talks.

To be correctly idiomatic while putting "is" at the end, one would write the second sentence:

I don't know which the correct one is.

Note "one" is a noun here.

More common and idiomatically correct would be to write:

I don't know which one is correct.

Note with regard to your first sentence, the answer to the question might be "The problem is X." While you would not state "The correct is X." because "is" requires a subject noun not just an adjective. An answer to the second type of question could be "The correct one is X."

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  • +1 (I would give you +2 for the 'Yoda" joke if I could.) I would add that there is a school of thought and English language training that states you do not end a sentence with "is". I doubt you would even be able to diagram any of these two sentences ending with that word. In writing, I would not end a sentence with "is", not even in an informal context (unless it is a direct quote). I may say a sentence with "is' at the end. However this would be for dramatic effect or I am reciting a direct quote. Otherwise, it is predominant in American usage and is widely accepted in various contexts.
    – Gary
    Mar 5, 2015 at 23:44
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It has to deal with the way verb phrases change when you have a question, as opposed to a statement. Let's look at a few examples:

What is the problem?

but

I don't know what the problem is.

Another example:

Who are these people?

but

My mother wonders who these people are.

What these examples show is that in a sentence that's a statement (not a question), words like "who", "what" aren't directly followed by a verb like they are in a question.

So let's try to work backwards from your second example, and see what the corresponding question would look like.

  • Statement: "I don't know which one correct is."
  • Question: we move the verb after 'which', and we get "Which is one correct?". That's not a valid English sentence, so there was a problem with our statement too.

There are two ways to ask that question:

Version 1: "Which is the correct one?"

Explanation: we start with "which is", and that looks like our familiar pattern of question word followed by a verb. The corresponding statement then looks like "I don't know which the correct one is", after we move the verb 'is' to the end. That's a valid statement.

or

Version 2: "Which one is correct?" (or "Which one is the correct one?")

Explanation: the question word 'which' doesn't have a verb right after it. In that case, the word order in the corresponding statement doesn't change. We get "I don't know which one is correct."

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