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I'm not a teacher
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What is the reason? - direct question
Do you know what the reason is? - embedded question (a question included in another sentence)
When you're dealing with an embedded question you need to change the word order to that of a statement (not a question).
Other examples of embedded questions:
- Could you tell me where I can find the bus station? (NOT *... where can I find the bus station?)
- I was wondering if you would like to go with me? (NOT *... if would you like to go with me)
- I would like to know where she is. (NOT *... where is she.)
- I don't know what she said. (NOT *... what did she say.)
He was absent. Do you know why?
He was late. Why? What is his reason for being late?
Do you know why = what is the reason that,
Never a need to write it twice in one sentence.