乃【nǎi】

乃 links a subject to what it is, like 是, but it belongs to literary and formal language: sayings, speeches, essays, inscriptions. It gives the sentence the weight of a judgement.

乃是 and 实乃

乃是 is a slightly fuller form. 实乃 "is truly" pushes the judgement further. Both stay in writing.

Nouns only, and no negative

What follows 乃 is a noun or noun phrase. It can't stand before an adjective, and it has no negative form – a 乃 sentence is always a statement.

• × 这个菜乃很好吃 → 这个菜很好吃
• × 他不乃学生 → 他不是学生

In conversation, 乃 sounds like a quotation or a joke; use 是.

乃至 "and even" is a separate word with its own entry. The old sense "thereupon" survives only in classical texts.