✅ Sentence 1 is correct: "If he knew it, I don't know what he would do." → Natural and clear. ❌ Sentences 2 & 3 are incorrect – Their word order is unnatural and confusing. Meaning: Sentence 1 expresses uncertainty about his reaction if he knew.

1 is best. You don’t necessarily need the “it”.

You could argue they are all grammatically correct. You could argue the punctuation is appropriate as it is. Some people would say stronger punctuation is needed, for clarity, especially for phrases 2 & 3; but I would counter that by saying the addition of extra punctuation would produce a potentially undesirable change in tone and potentially even a slight change in context. Personally, if writing for my own website for example, I would probably favour the double comma structure, for slightly increased clarity, since it also remains very open, so has minimal affect on tone: 2. If he knew it, he would do, I don't know what. 3. If he knew it, what he would do, I don't know. But if writing to impress an examiner, or writing for translation into another language, I would probably add much stricter punctuation, using a colon, for much more specific context, and suffer the slight change in tone, like this: 2. If he knew it, he would do: I don't know what. 3. If he knew it: what he would do, I don't know.

I think the first one sounds the most nature. With the second you need at the very least a comma but ultimately the second sounds clunky and awkward even if you could say it. The third - you could say that but again it is awkward. The third choice has the emphasis in the wrong order - only the first choice has the emphasis in the right order. You need to emphasis that you don't know and that's why what he will do is an unknown. Have a native speaker read the first line for you and you should hear that he/she will naturally emphasis "don't know."

If he knew it,I don't know what he will do.