merhaba herkes

I wrote to a new friend in Turkey saying "istanbul´da yasiyor musun?"

But he has said i should write "Istanbul´da mi yasiyorsun?"

Can anyone explain please?

The question particle MI can be added after any constituent of the sentence  -  the one you want to target your questioning at. So,

İstanbul´da mı yaşıyorsun? ´Is it Istanbul that you are living in (and not somewhere else)?´

İstanbul´da yaşıyor musun? ´Are you living in Istanbul (or just staying there temporarily for instance)?´

Sen mi İstanbul´da yaşıyorsun? ´Is it you who lives in İstanbul (and not someone else)?´

Abla

Yardimin icin tesekkur ederim

if you ask yes -no  question, you should use mi structure before the verb

if you ask yes -no  question, you should use mi structure before the verb

It is always a yes-no question when you use mi. So there is no need to use it before the verb unless you want to emphasize something.