得(děi, have to)
必须 (bìxū, must)
不用（bú yòng, don’t have to）
应该 (yīng gāi, should)
The differences between 得、必须 and 应该
得 (děi, have to) and 必须 (bìxū, must) both express obligation, and 得 (děi, have to)means a less strong obligation than 必须(bìxū, must).
得 (děi, have to) definitely necessary in order to do something. If not done, one is unable to do something.
Eg: 1、你发烧了，得去看医生。（Nǐ fāshāole, nǐ děi qù kàn yīshēng. you had a fever, you have to see doctor.）
2、我得赶快出发, 否则就迟到了。（Wǒ děi gǎnkuài chūfā, fǒuzé jiù chídàole. I have to set out in a hurry, otherwise I will be late.）
whereas 必须 (bìxū) would be equivalent to English must, is very sure, if you don’t do something that will cause serious consequences.
Eg. 你出国之前必须先办理签证手续。(Nǐ chū guó zhīqián bìxū xiān bànlǐ qiānzhèng shǒuxù. Before going aboard you must first obtain a visa. )
So 得 (děi, have to) and 必须 (bìxū,must) share the same mean. “必须 “is more formal than”得“. “得”Somewhat similar to 必须(bìxū, must) but much more colloquial. Actually “得” is a dialect in north China, it is often used in daily life and is not used in formal writing.
The negative form of必须 and 得 is不用(bú yòng), which would be equivalent to English don’t have to.
Yes, you can use 不必(bú bì, not necessary) too.不用=不必
Eg. 1、我明天不用早起（Wǒ míngtiān bú yòng zǎoqǐ, I don’t have to get up early tomorrow.）
2、我可以自己从机场回家,你不必来接我.( Wǒ kěyǐ zìjǐ cóng jīchǎng huí jiā, nǐ bú bì lái jiē wǒ, It’s not necessary to pick me, I can go home alone from the airport.)
“得” can also means “需要” (xūyào, need)
e.g.这辆车得多少钱?( Zhè liàng chē de duōshǎo qián? how much money needed to buy this car? )
In this sentence, you cannot use “必须” instead of “得”.
应该(yīng gāi) expresses necessity is similar to English should,
Eg. 我们应该(yīng gāi, should) 帮助穷人（we should help the poor.）
The negative form of 应该 is 不应该(bù yīng gāi, shouldn’t)
Eg. 他不应该对妈妈那样说话。（Tā bù yīng gāi duì māma nàyàng shuōhuà. He shouldn’t talk to his mother like that.）