Grammatical analysis of these two sentences
The sentence is derived as follows:
Istambul is a good city. (Declarative or statement form) Is Istambul a good city? (Yes/No interrogative) Yes, Istambul is a good city. (Positive reply/answer) No, Istambul is not a good city. (Negative answer) Isn't Istambul a good city? (Negative interrogative) When a question tag is added to it, it goes like "Istambul is a good citym, isn't it?"
This is a single-clause sentence with Istambul as the subject, IS, the verb, and the remaining make a complement phrase.