因为”香蕉很好吃“并不是一个完整的句子，它省略了谓语动词和宾语，香蕉（主语）很（程度副词）好吃（形容词是定语），它的完整形式是“香蕉是（谓语）很好吃的水果（宾语）。” 很多判断句都可以省略谓语和宾语，并改变定语的形态（去掉的），如：”他很高大。“完整是”他是很高大的人。“/ ”树叶很绿。“完整是”树叶是很绿的颜色。“ 因此并不是因为没有”是“，而是省略了。 实际上如果想要加上”是“，只需要把定语的形容词变回原形，即”香蕉是很好的。“，又如：”她很漂亮。“变为”她是很漂亮的。“ 在口语情况下即使在原句加上”是“，这个句子也是通顺的，但是语意就改变了，在于说的重音在哪里，当使用香蕉 ‘ 是很好吃时，意味着强调定语”好吃“，而如果说香蕉是' 很好吃时，则是强调程度副词”很“。 =============================== In case you don't understand what I mean thoroughly, I will explain it via English as best as I can. The sentence you informed is not a complete one in fact, it has abbreviated the component of predicate and object. 香蕉（noun as sub.)很（advert of degree)好吃（adj.as attribute)。 And the complete form of this sentence is "香蕉是(verb as predicate)很好吃的水果（noun as object)。" Many sentences in Chinese can be structured in form as this; abbreviating their predicate and object simultaneously vary their adjective's form by erasing "的",for instance, ”他很高大。“is actually”他是很高大的人。“/ ”树叶很绿。“in its complete form is”树叶是很绿的颜色。“ Thus it is not like there is no need to use "是”，but is abbreviated. In fact, if you'd like to includ "是" in your sentence, the only thing you have to do is to change the adjective back to its original form by adding "的" after it, as ”香蕉是很好的。“，another example in case is:”她很漂亮。“ can shifts to”她是很漂亮的。“ Additionally, in oral context it is not a lapse to add "是" in the sentence you informed, only the changing of meanings which depend upon your intonation, when saying "香蕉' 是很好吃（stress 是）,you might mean to make you point clear that it is true to state banana is delicious, in other words, you are emphasizing the adjective, but notice, commonly in circumstance like this, there will always has a negative comment following. On the other side, in situations by enunciating "香蕉是' 很好吃"(stress 很) always indicates that you want to express how delicious the banana is, which means you are emphasizing the advert. I've tried my best already, but if you still feel confused, please feel free to inform me as long as you don't think I'm too messed up.And if only you can kindly help me by pointing out the grammar mistakes I might have committed in English?

We usually don't use "be" in Chinese when you say sentence with “很”.For example,you can also say “天气很热”（It is very hot today）“你很好”（You are so kind）.

如果你要用“是”，就必须说“香蕉是很好吃的”。人们还是习惯说“香蕉很好吃”。

in fact,it's a manner in chinese.

I have no idea about grammar rule for your question, but my versions for both questions in English are "Bananas taste good./ Bananas do taste good." Here, I can give you a context to use "香蕉是很好吃" . ex. 香蕉是很好吃，但是不能吃多了。 by the way, you could just think of 吃 as a verb in this case, it's correct and easy to memerise.