The adverbs 一定 and 千万
一定 and 千万 are modal adverbs (语气副词). They both express subjective judgment or assurance toward an action. Their meanings are similar, but we use them in different situations.
The adverb 一定
The adverb 一定 
has a connotation of determination, or obligation, and is often used when issuing a command.
In the sentence, it should come before the predicate.
has a connotation of determination, or obligation, and is often used when issuing a command.
In the sentence, it should come before the predicate. Pay attention
It is often followed by the modal verb 要 .
.Scheme
一定
predicate
Examples
你一定要让老师参加这个活动
You definitely should convince the teacher to participate in this activity
nǐ yídìng yào ràng lǎoshī cānjiā zhè ge huódòng
你看这些鞋子，给你一定会很合适
Look at these shoes. They definitely will suit you very well
nǐ kàn zhèxiē xiézi, gěi nǐ yídìng huì hěn héshì
我一定要八点前回家
I must be home by 8:00
wǒ yídìng yào bā diǎn qián huí jiā
The word 一定 can also act as the adjective “certain”. In this case we should put the particle 的 between the adjective and the object.
between the adjective and the object.Examples
你们需要办理一定的手续
You need to go through certain procedures
nǐmen xūyào bànlǐ yídìng de shǒuxù
To make the negation, negative particles can come either before 一定: “not necessarily doing something”, or after 一定: "necessarily not doing something."
Examples
他的看法不一定是跟我们一样的
His point of view is not necessarily the same as ours
tā de kànfǎ bù yídìng shì gēn wǒmen yíyàng de
太太一定不答应
Madame will certainly never agree
tàitài yídìng bù dāyìng
一定没认出来吧？
You must not have recognized (him), right?
yídìng méi rèn chūlái ba
The adverb 千万
The adverb 千万 has a similar meaning. However, it has a less bossy connotation than 一定.
It is used with a connotation of insistent request rather than obligation.
has a similar meaning. However, it has a less bossy connotation than 一定.
It is used with a connotation of insistent request rather than obligation. Pay attention
Most often, 千万 is found in negative sentences. It may be followed by the negative particles 别 or 不 .
or 不 .Examples
千万别忘了
Don’t ever forget
qiānwàn bié wàng le
千万不要告诉他
Never tell him
qiānwàn bùyào gàosu tā
机会太好了，你千万不能错过
The opportunity is very good. Don't miss out on it by any means
jīhuì tài hǎo le, nǐ qiānwàn bù néng cuòguò