1. Explain it simply
不得不 puts two negatives together, and they cancel into a strong 'have to': S + 不得不 + verb phrase.
It shows an obligation forced by circumstance, not a free choice: 下大雨了，我们不得不打车回家.
Both 不 are required; the meaning comes from the pair working together.
State the reason first, then 不得不 + the action you are pushed into.
2. Show examples
下大雨了，我们不得不打车回家。
Xià dà yǔ le, wǒmen bùdébù dǎchē huíjiā.
It is raining hard, so we have no choice but to take a taxi home.
circumstance forces the taxi
我生病了，不得不请假。
Wǒ shēngbìng le, bùdébù qǐngjià.
I am sick, so I have no choice but to ask for leave.
being sick forces the leave
太晚了，我们不得不回家。
Tài wǎn le, wǒmen bùdébù huíjiā.
It is too late, so we have no choice but to go home.
the late hour forces it
3. Share a tip
Keep both 不 together - say 我们不得不打车, not 我们不得打车。