很 + 形容词【hěn + xíngróngcí】

An adjective can be the whole predicate of a sentence. It takes no verb in front of it, and the slot where English puts "is" is filled by 很.

是 does not go before an adjective

是 links the subject to a noun, and 忙 is not a noun, so 是 has nothing to do here. English "I am busy" and "I am a student" look alike, but here they are two different sentence types:

• × 我是忙 → 我很忙
• × 她是漂亮 → 她很漂亮

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: 我不忙. 不很忙 does exist, but it means "not very busy", and 不太忙 is the usual way to say that.

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.