If you look up 肯定 (kěndìng), 确定 (quèdìng) and 一定 (yīdìng) in the dictionary, they all have pretty close meanings, like "certainly" or "surely". However, using them correctly in a sentence is a bit more complicated.
So how are these three words different? We can answer this question from two points of view:
From a part of speech point of view:
肯定 can be a verb, an adjective and an adverb. 确定 can be a verb or an adjective. 一定 can be an adjective or an adverb.
From a significance or meaning point of view:
肯定 often has a subjective, energetic feel to it. 确定 carries a feeling of objectively describing a situation. 一定 as an adjective has really broad uses, so you need to pay attention to the specific times it's used in. As an adverb, it is often used to give a reinforcing tone.
"肯定" and "确定" as a Verb
肯定 and 确定 can both be verbs, but their use and meanings are different. 一定 cannot be used as a verb.
肯定 can express that something is satisfactory, often used when a superior evaluates subordinates. When used this way, it should be followed directly by a noun or a phrase that is acting as a noun.
肯定+ Noun/Noun Phrase
It can also be used to express "I am sure". When used this way it is usually a clause at the end of a sentence.
它也可以用于表达"I am sure"。这种情况下，句子的结尾一般是一个从句。
Subject + 肯定 + sentences
lǎo bǎn kěn dìng le wǒ men tuán duì de gōng zuò.
The boss approved our team's work.
rú guǒ xiǎng ràng bié rén kěn dìng nǐ, nǐ jiù yào hǎo hǎo zuò.
If you want to make other people like you, you have to do good work.
wǒ gǎn kěn dìng zhè jiù shì tā de shēng yīn.
I'm sure that this is his voice.
You can add a verb phrase or a preposition phrase after 确定, which makes it mean that something is already set, often used in describing a progressing condition.
确定 + Phrases / short sentences
wǒ men hái méi yǒu què dìng shén me shí hòu kāi shǐ.
We still haven't decided when to start.
nǐ què dìng tā shì wǒ men yào zhǎo de rén ma?
Have you decided if he's the person we'll look for?
"肯定" and "一定" as an Adverb
肯定 and 一定 can both be adverbs, and the grammar structure for both are similar. 肯定 emphasized more of a subjective judgment or promise. 一定 sometimes carries an air of forcing something or issuing a command.
肯定 + Adjective / Verb
tā kěn dìng hái méi qǐ chuáng, yīn wéi tā de shǒu jī xiàn zài hái méi kāi jī.
He definitely hasn't gotten out of bed yet, because his phone is still off.
wǒ kěn dìng huì gěi nǐ dài lái zhè bǐ qián de.
I will definitely bring you the money.
一定 + Adjective / Verb
míng tiān nǐ men yī dìng yào lái.
You guys are definitely going to come tomorrow.
yī dìng yào xiǎo xīn.
You've definitely got to be careful.
"肯定", "确定" and "一定" as Adjectives
All three of these can be placed before a noun as adjectives. But in this grammar structure, 肯定 and 确定 use their verb meanings, and 一定 as an adjective means "something particular".
肯定 + (的 +) Noun
确定 + (的 +) Noun
一定 + (的 +) Noun
nǐ kěn dìng de shì qíng shì shén me?
What do you know for sure?
qǐng gěi wǒ yī gè què dìng de huí dá, bú yào zǒng shì zhè me mó hú.
Please give me a definite response. Don't always be so vague.
wǒ men zhī jiān yào bǎo chí yī dìng de jù lí.
We need to keep a certain distance between us.
zhè jiàn shì hé tā yǒu yī dìng de guān xì.
He has a certain connection with this.
Translated from: allsetlearning
Please visit Chinlingo for Chinese learning.