"मैं लड़का नहीं हूँ।"
अनुवाद:I am not a boy.
'I am not boy' is not a valid English sentence, it is simply incorrect. The correct sentence would be 'I am not a boy'. When translating you must avoid translating literally word for word because this will not give you valid sentences! एक is allowed to be dropped in the Hindi sentence, but you must not drop 'a' from the English sentence :)