We are to make the sentence that has this answer: Benim kitabım evde kaldı. The correct answer is Senin kitabın evde mi kaldı? But why is it not Senin kitabın evde kaldı mı? ? Thanks!

 It has something to do with the important word in the sentence and in this sentence the word at home is the word that is being stressed more than your book (I think)

but I still can`t get my head round the correct position

I think it has something to do with the position of ´mi´. They´re asking ´was it at home(as opposed to somewhere else)that it was left?´ rather than ´was it left at home?´ The difference is fairly subtle.

If the ´mi´ had been after ´kitabın´ the question would be ´was it your book left at home?´ (rather than something else)

My try