Modal verbs with the meaning "must": 得, 需要 and 必须
Modal verbs (情态动词) can be used to express possibility, desire, ability, and necessity. Modal verbs cannot appear without a predicate and can only appear in predicate phrases.
The verb 得
The modal verb 得 
is most commonly used in colloquial speech and indicates either that the speaker wants an action to be done, or that the action has been scheduled.
is most commonly used in colloquial speech and indicates either that the speaker wants an action to be done, or that the action has been scheduled.Examples
我得去北京出差
I have to go to Beijing on business
wǒ děi qù běijīng chūchāi
你得早起
You have to get up early
nǐ děi zǎo qǐ
得 can be placed before a numeral and express either an estimate or an assumption.
Examples
这个设计得6个月才能完
This design will take 6 months to complete
zhè ge shèjì děi liù ge yuè cái néng wán
得 is only used in affirmative sentences.
The negation is formed by using 不必 and 不用 .
甭 can be used in colloquial speech.
and 不用 .
甭 can be used in colloquial speech.
Pay attention
不得 cannot be used.
Examples
你明天不必早起
You don't have to get up early tomorrow
nǐ míngtiān búbì zǎoqǐ
甭着急
Don't worry
béng zháojíbéng zháojí
Pay attention
The negative form of modal verbs is formed by adding the negative adverb 不. The negative adverb 没 cannot be used here.
cannot be used here.The verb 需要
需要 expresses a need that is caused either by intrinsic motivation or by external conditions.
It is commonly used in formal language.
需要 can be followed either by a verb phrase or a noun.
expresses a need that is caused either by intrinsic motivation or by external conditions.
It is commonly used in formal language.
需要 can be followed either by a verb phrase or a noun.
Pay attention
A verb phrase is a combination of a verb and an auxiliary word. The auxiliary word can be either an auxiliary verb or an object.
Examples
她需要治疗
She needs treatment
tā xūyào zhìliáo
大家需要氧气
Everyone needs oxygen
dàjiā xūyào yǎngqì
You can also use 需 instead of 需要. However, 需 must be followed by at least four characters.
Examples
父亲的身体需休养一段时间
Father needs some time to recover
fùqin de shēntǐ xū xiūyǎng yí duàn shíjiān
If 需 is preceded by an adverb, it can be followed by less than four characters.
Examples
我们只需遵从
We just have to obey
wǒmen zhǐ xū zūncóng
The verb 必须
必须 expresses the need to do an action. There are two instances when you can use this verb:
1) when there is no other choice in the situation.
expresses the need to do an action. There are two instances when you can use this verb:
1) when there is no other choice in the situation.Examples
护士必须穿制服
Nurses must wear uniforms
hùshì bìxū chuān zhìfú
哥哥不能跟我们一起去，他必须做完作业
My brother can't come with us because he has to finish his homework
gēge bù néng gēn wǒmen yìqǐ qù, tā bìxū zuòwán zuòyè
2) when someone commands people to do something.
Examples
你必须告诉他
You have to tell him
nǐ bìxū gàosu tā
你们必须听我的话
You must listen to me
nǐmen bìxū tīng wǒ de huà
必须 can serve as an attribute. The attribute is usually followed by the particle 的 .
.Examples
有些事是必须的
Some things are necessary
yǒuxiē shì shì bìxū de
The negation is formed by using 不必 and 不用.
Examples
你不必今天做完这项工作
You don't have to finish this work today
nǐ búbì jīntiān zuò wán zhè xiàng gōngzuò