不得不 (have no choice but to)

No other option

• Your friend has a high fever on the morning of an important exam.
• he他tāto have a high temperature…发烧fā shāo(completed / change)了le，have to不得不bù dé bùto request leave of absence请假qǐng jià。 — "He has a fever, so he has no choice but to ask for leave."

Subject + have to不得不bù dé bù + Verb

• have to不得不bù dé bù = "have no choice but to, be forced to": outside pressure makes the action necessary.
• It sits after the subject, before the verb: I我wǒhave to不得不bù dé bùadmit承认chéng rèn, we我们wǒ menhave to不得不bù dé bùto change改gǎiplan计划jì huà.

I我wǒhave to不得不bù dé bùadmit承认chéng rèn，he他tāsays说shuō得deright对duì。

Rules, money, health

• because因为yīn wèinot / haven't没méicoin钱qián，he他tāhave to不得不bù dé bùto sell off卖掉mài diào(completed / change)了lewar chariot (archaic)车chē。 — "Because he had no money, he had to sell his car."
• the flight飞机fēi jīdelayed晚点wǎn diǎn(completed / change)了le，we我们wǒ menhave to不得不bù dé bùat在zàithe airport机场jī chǎngstay住zhùone night一晚yì wǎn。 — "The flight was delayed, so we had to spend a night at the airport."

the flight飞机fēi jīdelayed晚点wǎn diǎn了le，we我们wǒ menhad to不得不bù dé bùat在zàithe airport机场jī chǎngstay住zhùone night一晚yì wǎn。

Not "must not"

have to不得不bù dé bù versus to have no other option but…只好zhǐ hǎo and must必须bì xū

• to have no other option but…只好zhǐ hǎo is resigned and casual: 没车méi chē(completed / change)了le，to have no other option but…只好zhǐ hǎoto walk走zǒuto return回去huí qù.
• must必须bì xū states a requirement: you你nǐmust必须bì xūbring带dàiyour passport护照hù zhào. have to不得不bù dé bù stresses being forced by the situation.

you你nǐmust必须bì xūbring带dàiyour passport护照hù zhào。

Recap

Read the full reference: HSK 4 grammar — Adverbs.