不得不 (have no choice but to)

"I have no choice"

• have no choice or option…不得不bù dé bù = "have to / cannot but" — a double negative that means "must".
• down下xiàheavy rain大雨dà yǔ，I / me我wǒhave no choice or option…不得不bù dé bùto take a taxi (in town)打车dǎ chē = "it poured, so I had to take a taxi."
• Two negatives (not不bù…not不bù) cancel into a forced "must".

Subj + have no choice or option…不得不bù dé bù + Verb

• have no choice or option…不得不bù dé bù = forced to, with no alternative.

I我wǒhave to不得不bù dé bùleave走zǒu了le。

• I / me我wǒhave no choice or option…不得不bù dé bùto walk走zǒu(completed / change)了le = "I have to leave (no choice)."

have no choice or option…不得不bù dé bù vs to have to必须bì xū

• have no choice or option…不得不bù dé bù stresses outside pressure (forced); to have to必须bì xū is a plain "must".

Recap

Read the full reference: HSK 4 grammar — Adverbs.