乃（＝是）
is; is indeed (written, with classical weight)
Structure
A ＋ 乃 ＋ B（名词短语：定义、判断）
What it means
乃 (nǎi) is classical Chinese for 是: it links a subject with a noun phrase that defines or judges it, with classical weight.
How it is built
A + 乃 + noun phrase: 失败乃成功之母 (failure is the mother of success). Also 胜败乃兵家常事, 此乃天意. 乃是 is also used and just as formal.
When to use it
Formal writing, speeches and set phrases. In conversation it sounds bookish or playful: 此乃本店的招牌菜. Don’t confuse it with 乃至 “and even”.
Watch out
Only a noun phrase follows 乃: ✗ 他乃很高 ✗ 他乃喜欢昆曲. The negative is 并非 or 非, never ✗ 不乃.
Examples
昆曲乃百戏之祖，京剧等许多剧种都从它那里汲取过营养。
Kunqu is the ancestor of all Chinese opera: Peking opera and many other forms have drawn nourishment from it.
失败乃成功之母，这次没考好，下次再努力。
Failure is the mother of success: you didn’t do well this time, so try harder next time.
此乃本店的招牌菜，各位一定要尝一尝。
This, ladies and gentlemen, is the house speciality: you really must try it.
Learn this in Lesson 240 · HSK 9+: 昆曲《牡丹亭》 (Kunqu and The Peony Pavilion)
A dialogue, a reading, word cards with audio and a drill that uses it, in Learn Chinese with Bao. The first lessons of every level are free.
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