不亦…乎
is it not …? a rhetorical question meaning “it truly is”
Structure
不亦 形容词 乎 ？
What it means
Is it not …?: a classical rhetorical question that really means “it truly is …”.
How it is built
不亦 + adjective + 乎？: 有朋自远方来，不亦乐乎？ (To have friends come from afar: is that not a joy?)
When to use it
亦 is 也 and 乎 is 吗, so it reads 不也…吗？. Found in classical texts and quoted in speeches. Today 不亦乐乎 is also an idiom for “to the utmost”: 忙得不亦乐乎 (rushed off their feet).
Watch out
In 不亦说乎, 说 is read yuè and means 悦, pleased, not “speak”. Don’t build new phrases with it; modern Chinese uses 不是很…吗？.
Examples
有朋自远方来，不亦乐乎？
To have friends come from afar: is that not a joy?
学而时习之，不亦说乎？
To learn and practise it in due time: is that not a pleasure?
Learn this in Lesson 206 · HSK 9+: 文言入门：之乎者也 (Classical Chinese: the four little words)
A dialogue, a reading, word cards with audio and a drill that uses it, in Learn Chinese with Bao. This lesson is free.
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