"Why are you not eating?"
Can we use 'niye' in informal conversatoin?for example Niye buni yapdin?
Lütfen, niçin "Neden sen yemiyorsun?" yanlış? Please, why is "Neden sen yemiyorsun?" wrong?
it is not wrong, it is a forgotten alternative. However you should keep in mind that it wouldn't be used often. It emphasizes the subject "SEN"
Thanks! But is this a characteristic of Wh- questions exclusively or a general rule (i.e., whenever a subject can be omitted, it normally is)?
yes the subject is often omitted. But more important thing is that whatever comes before the verb is emphasized. So if you said "sen neden yemiyorsun?", you wouldn't really emphasize the subject. In Turkish the question words are placed where their answer would be, they don't need to be placed in the beginning of a sentence like in English.