很 + 形容词【hěn + xíngróngcí】

An adjective works as the whole predicate in Chinese. It needs no verb in front of it, and the slot where English puts "is" is filled by 很.

是 does not go before an adjective

wǒ我shì是máng忙 is wrong. 是 links two nouns; 忙 is not a noun, so there is nothing for 是 to link. Say 我很忙.

The mistake survives translation because English "I am busy" and "I am a student" look identical. In Chinese they are two different sentence types:

A bare adjective sounds like a comparison

Drop 很 and the sentence stops being a plain description. 今天冷 implies "today is cold – as opposed to yesterday", and a listener waits for the other half.

Negation and questions

不 replaces 很 rather than joining it. wǒ我bù hěn不很máng忙 is not how this is said – 我不忙 is.

Questions take 吗, or repeat the adjective around 不.

Other degree words replace 很

很 is only the default filler. Swap it for 非常, 真 or 太…了 and the degree becomes real.