我不是很好 = I am not feeling so good;
I don't really use 我不是很好 or 我不好 because I hardly say that or hear people say that in real life, it's a bit unnatural to include the subject "我";
What I use the most to negatively respond to 你好吗？ is 不太好 which also means not really good but this one sounds more natural/casual by leaving the subject out. Be sure to ...
我不是很好 means I'm not feeling quite well, but not that bad.
我不好 literally means I'm feeling bad. But this sounds a bit rude/impolite. When you're unhappy and not likely to have a conversation with the guy, you can say this. However, when you are very familiar with the guy, you can say this with a joking tongue.
我很不好 means you are feeling quite bad.
Seriously speaking, there is nothing wrong with either 这太棒了！, 这是太棒了！ or 这真是太棒了！ If we consider the three sentences/phrases below, the middle one seems incomplete as one would wonder "what is too good?", while the other two are simply arbitrary exclamations without the need to address "what is (是). However, this argument is weak, so your friend'...
As a native user, I'll try my best to explain these questions.
Here should be a special grammar point in Chinese. On building a sentence to describe a none, especially to show surprise, we make something like [n + adj]. Mostly, it is unnecessary to insert a BE verb.
In the 1st and 3rd sentences, the structures are like ...