I know the other poster understood you, I'm trying to say that the other poster might assume you're right in every situation when that's just not the case. Since they provided the context, that being they're studying Chinese, when they say 不是很好 (I'm not very good), it's correct because the context serves as a noun of sorts (referring to their standard). You try saying stuff like 我很不好 (I'm very not good), you'd be laughed at in a conversation. Other exceptions like the question: 我是不是很胖? (Am I or am I not fat?) is another example we can use.
I know the other poster understood you, I'm trying to say that the other poster might assume you're right in every situation when that's just not the case. Since they provided the context, that being they're studying Chinese, when they say 不是很好 (I'm not very good), it's correct because the context serves as a noun of sorts (referring to their standard). You try saying stuff like 我很不好 (I'm very not good), you'd be laughed at in a conversation. Other exceptions like the question: 我是不是很胖? (Am I or am I not fat?) is another example we can use.