This question already has an answer here:
Very early in the process of learning Chinese as a foreign language we are taught:
- 我是好 is incorrect because 是 is only used before nouns and 好 is not a noun.
- 我很好 is the correct way to say the former
- 很 literally means "very" but in this type of usage doesn't have this literal meaning.
- 很 isn't always used between a noun and an adjective
(This naturally results in some foreign learners thinking of 很 as some kind of special equivalent to "to be".)
But when should we use 很 and when should we omit it? In some other places in Chinese grammar we use some character with monosyllabic words and omit it with bisyllabic words. I'm not sure if that's the case here though since I'm sure I've seen 很 used with both monosyllabic and bisyllabic words.