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yí dìng
adv./adj. surely, definitely, certainly; fixed, specified; certain, particular; proper, due, fair
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In this post, we discuss difference between 一定 and 准 when they are both adverbs:

准 adv. definitely, surely
一定 adv. surely, definitely, certainly

Here are some points that will help you determine when to use which. We hope this is helpful to you, but if in any case we missed anything, leave a comment below. We appreciate your feedbacks!

1. We can add 不 before 一定 to form a negative sentence, but we cannot do the same to 准.
Example 1a. 失败不一定是坏事。 In this sentence we cannot use 准 to replace 一定.

2. When expressing a subjective will, promise or resolution, we use 一定.
Example 2a. 明天我一定会守时的。 [phr] I'm sure I will be on time tomorrow.
Example 2b. 我们一定要坚持下去,不能半途而废. [phr] We should continue and persist on, instead of abandoning it halfway.

3. When expressing a subjective estimation or guess, we use both 一定 and 准.
But 一定 can be used both before actions/events (verbs or verbal phrases) and states (adjectives), as demonstrated by the examples below; 准 cannot be used before adjectives as demonstrated in 3b.
Example 3a. 如果他不在宿舍的话,准是去图书馆了。 [phr] If he is not in the dormitory, he is surely in the library. (Or we can say"...,一定是去图书馆了。")
Example 3b. 现在是夏天,一定很热吧? [phr] Now it's summer. It's definitely quite hot, right? (we cannot use 准 to replace 一定)

4. When making a statement about a fact, a suggestion or an instruction, we only use 一定.
Example 4a. Suggestion: 来北京一定要尝一尝北京的小吃。 [phr] (If you) come to Beijing, (you) must taste the Beijing snacks.
Example 4b. Instruction: 离开房间的时候,一定要关闭电源。 [phr] Be sure to turn off the electricity when you leave the room.
Example 4c. Fact: 人一定会有缺点,没有人是十全十美的。 [phr] People all have shortcomings, no one can be perfect.
肯定 vs 一定

1. 肯定 and 一定 have different parts of speech, but they can both be adverbs and mean "definitely" -- in this meaning, they are basically the same. For example, this sentence below, we can also use 一定:
你送他礼物,他肯定会很高兴的。 [phr] (If) you give him a gift, he will certainly be very happy.

2. 肯定 and 一定 can both be adjective, but their meanings are different. 肯定 means "positive, affirmative"; "一定" means "fixed, specified; certain, particular; proper, due, fair".
E.g. 肯定的评价 [nphr] positive assessment
E.g. 斑马身上的斑纹有一定的保护作用。 [phr] The stripes on the body of a zebra have a proper protecting function.
E.g. 一定之规 [idiom] fixed pattern, set rule; one's own way of doing things

3. 肯定 can be a verb, while 一定 cannot be a verb.
E.g. 你能肯定吗? [phr] Can you be sure?
What is the proper verb tense when using 一定 (yídìng)?
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Are there any synonyms or antonyms of 一定 (yídìng) that could be used instead?
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