1. Do you know? + Can he play the piano?
2. Do you know if/whether he can play the piano?
3. Do you think? + Is he still alive?
4. Do you think if he is still alive?
* 4-1. If do you think he is still alvie?
5. Do you think he is still alive?
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Would you check the sentences? Which ones are grammatical or ungrammatical?
2. OK
5. OK
4 and 4-1 are not OK.