Learn Italian: Il traffico era terribile, perciò ci siamo dovuti fermare. - The traffic was terrible, so we had to stop.

This is the core rule of this lesson! The main verb here is the reflexive fermarsi (to stop oneself). Because we placed the reflexive pronoun ci before the conjugated verb phrase (ci siamo dovuti...), the auxiliary verb must switch to essere (siamo).

Why does the past participle change to dovuti instead of staying dovuto?

Whenever you use essere as your auxiliary verb in the passato prossimo, the past participle has to agree with the subject in gender and number. Since the subject is an implied 'we' (noi), dovuto becomes the plural dovuti. If 'we' referred to an all-female group, it would be dovute.

Could I attach ci to the end of the infinitive instead?

Yes, you can say fermarci. However, if you attach the pronoun to the end of the infinitive, the modal verb goes back to its normal auxiliary (avere), and the past participle no longer agrees with the subject. The clause would become: ...perciò abbiamo dovuto fermarci. Both versions are completely correct and common!

Why is era in the imperfect tense but siamo dovuti fermare is in the passato prossimo?