未必
Reach for this when you want to push back on someone's confident assumption without being blunt. 未必 means 'not necessarily,' and it's the literary, slightly cautious cousin of 不一定. Use it to say 'you think X is true, but it isn't guaranteed.' Common in essays, op-eds, and thoughtful spoken Chinese where the speaker wants to keep the door open instead of saying flatly 'no.'
Structure
[SUBJECT] 未必 [VERB / ADJ]
wèibì
How to Think About It
Break it down: 未 = 'not yet' (future-tinted), 必 = 'must, certainly.' So 未必 literally is 'not yet certain' — leaving room for the opposite to be true. It's softer than 不会 ('won't') and more literary than 不一定. Native speakers reach for it when they want to sound measured: 'you may be right, but I'm not sure.' Stress falls on the implied counterpossibility.
Examples
你以为不可能的事情，未必真的不可能。
Nǐ yǐwéi bù kěnéng de shìqing, wèibì zhēn de bù kěnéng.
Things you think are impossible aren't necessarily really impossible.
贵的东西未必好。
Guì de dōngxi wèibì hǎo.
Expensive things aren't necessarily good.
他答应了，未必会做到。
Tā dāyìng le, wèibì huì zuò dào.
He agreed, but he won't necessarily follow through.
Common Mistake
Learners attach 未必 to the wrong half of the sentence. 未必 goes before the verb or adjective that's being doubted, not at the end. '他来未必' is wrong; '他未必来' is right.
你以为他赢，他赢未必。
你以为他会赢，他未必会赢。
Don't Confuse With
不一定
不一定 is the everyday colloquial version. 未必 sounds more bookish and is favored in essays and careful speech.
未必不...
未必不 = 'not necessarily not' = 'might well.' Double negative softens the affirmation. Don't conflate with bare 未必, which doubts a positive claim.
也许 / 可能
也许 / 可能 introduce possibility neutrally ('maybe X'). 未必 specifically pushes back on someone's assumed certainty ('not necessarily X').
Practice
贵的东西 ___ 好。
Show answer
未必
你以为不可能的事情，未必真的 ___ 可能。
Show answer
不
Arrange: 未必 / 答案 / 这个 / 正确
Show answer
这个答案未必正确。
Hard work doesn't necessarily lead to success.
Show answer
努力未必能成功。
Push back on a common assumption using 未必.
Show answer
看起来简单的事，做起来未必容易。
Hear It in Real Episodes
This pattern appears in 1 Fluentide episode: