一定
certain; definitely
yídìng
What does 一定 mean?
一定 (yídìng) is a high-frequency adverb meaning 'definitely, certainly, for sure' — used to express strong certainty about what is, was, or will be true. It can also function as an adjective meaning 'a certain (amount of), a definite' (一定的经验 'a certain amount of experience'). Two things English speakers often miss. First, 一定 expresses the speaker's confidence, not external proof — 他一定来 means 'he will definitely come (I'm sure)', not 'it has been confirmed he will come'.
Second, in promises and commitments 一定 is the standard way to make a vow: 我一定来 ('I'll definitely be there'), 我一定记得 ('I'll be sure to remember'). The negative form is 不一定 ('not necessarily'), which is itself one of the most useful expressions in spoken Chinese. Note the tone change: 一 is yì by default but becomes yí before a fourth tone (dìng).
Character breakdown
one; fixed
to set; fixed; certain
Memory hook: 一 (one, fixed) + 定 (settled) = one settled answer = definitely.
Example sentences
我一定来。
Wǒ yídìng lái.
I'll definitely come.
spoken
你一定要小心。
Nǐ yídìng yào xiǎoxīn.
You absolutely must be careful.
spoken
他不一定知道。
Tā bù yídìng zhīdào.
He doesn't necessarily know.
neutral
明天一定会下雨。
Míngtiān yídìng huì xià yǔ.
It will definitely rain tomorrow.
neutral
这件事我们一定要做好。
Zhè jiàn shì wǒmen yídìng yào zuò hǎo.
We must do this thing well, no matter what.
neutral, written
Common phrases with 一定
Synonyms
肯定 also means 'definitely, certainly' and is largely interchangeable with 一定 as an adverb of certainty. 肯定 sounds slightly more spoken-emphatic ('I'm positive'), while 一定 is more neutral and is the preferred choice in promises (我一定到, 'I'll definitely be there').
Don't confuse 一定 with
FAQ
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