Negatives are tricky in any language, and apparently all the more so in English. But they shouldn´t be; at times, it´s just because English is all over that some of its simplest uses turn into headaches.
No and not are two of the most common English words to express negation. And it´s as simple as this:
Long story short, no means no. The longer version: no is used when the answer to a question is negative:
Are you hungry? No, I am thirsty.
No is also used before a noun or a noun phrase:
My mom always said: No sweets before dinner!
I have no clue what to do now.
And since there is no other choice, not is used with all the other words or phrases:
I am not hungry, I am thirsty.
Mary does not know why she was called in the principal´s office.
“Do you read any books?” “Not science-fiction”.
“Students found this exercise pretty difficult. Did you?”. “No, not at all”.
It´s that easy! But for those of you who want to make things more complicated, watch this video about the origins and uses of none .