乃 / 即: is, namely (written copula)
动词 / verbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
A + 乃 + B (= A 是 B) | A，即 B (= that is)
Two classical copulas replacing 是 in writing:
• 乃 = "is (truly)", with solemn affirmation: 失败乃成功之母 (failure is the mother of success).
• 即 = "that is, namely": used to define or specify: 知行合一，即认识和实践相统一.
Also 即是、乃是. Typical of philosophical texts and maxims.
Examples
失败乃成功之母。
Shībài nǎi chénggōng zhī mǔ.Failure is the mother of success.
“知行合一”，即认识和行动要统一。
“Zhī xíng hé yī”, jí rènshi hé xíngdòng yào tǒngyī."Unity of knowledge and action": that is, understanding and action must be one.
孔子乃中国历史上最有影响的思想家之一。
Kǒngzǐ nǎi Zhōngguó lìshǐ shang zuì yǒu yǐngxiǎng de sīxiǎngjiā zhī yī.Confucius is one of the most influential thinkers in Chinese history.
Common mistakes
✗ 乃 in plain self-introduction: ✗ 我乃学生 (sounds like a martial-arts film) → ✓ 我是学生.
✗ Confusing 即 (namely) with 既 (since / both).