what is difference between 很，非常?

both means "very, so, very much", but it normally goes with some fixed expression.
you can say: special moment/period: 非常时期。 BUT you cannot say" 很时期”。

很 and 非常 can mean "so, very, quite..." normally 很+adj or非常+adj, eg.很好，非常好，很聪明，非常聪明
Difference:很can be in the structur like adj+得很，eg.好得很，聪明得很， but 非常 doesn't have such kind of structure

很 and 非常: their similarity is both of them mean"very, quite, extremely". Both of them is an adverb.
They could modify adjctive. For example: 很漂亮，非常漂亮

Their difference is : 非常 could be a Adjective. It could modify noun.
For example: 非常时期（special period, unusual period）
but we can't say: 很时期

很 and 非常 both mean "very", but a little bit different.

很 is an Adverb, which means "very, quite, extremely"
非常 is an Adverb is also an Adjective, which means "very, quite, extremely, special, unusual"

很满意 (OK)
非常满意 (OK)
very satisfied

很高兴 (OK)
非常高兴 (OK)
very happy

很努力 (OK)
非常努力 (OK)
very hard

好得很 (OK)
非常得很 (NOK)
very good

很不错 (OK)
非常不错 (NOK)
very good

很时期 (NOK)
非常（的）时期 (OK, as Adj.)
unusual/special times

很行动 (NOK)
非常（的）行动 (OK, as Adj.)
special action

I can tell one difference, They both express degree, but "非常" is more formal and the degree is higher than "很". For example, if I chat with a friend about a movie, I can say”很精彩““太好看了”。 if I talk to a parent that his kid studies hard at school, I can say"他非常努力"(the degree is greater than "他很努力"