Learn Italian: Siamo dovuti tornare a casa presto.

When a modal verb like dovere (to have to) is followed by another verb in the infinitive, it usually borrows the auxiliary that the second verb would normally use. Because tornare (to return) uses essere (to be) in the past tense, the modal verb takes essere here too.

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

Because we are using the auxiliary verb essere (siamo), the past participle must match the subject in gender and number. Since the subject is 'we' (a plural group), dovuto changes to the plural form dovuti. If 'we' referred to a group of only women, it would be siamo dovute.

Is it completely wrong to say abbiamo dovuto tornare instead?

Not completely! In modern, everyday spoken Italian, it is becoming very common to just use avere with modal verbs, regardless of the second verb. However, matching the auxiliary to the second verb (siamo dovuti tornare) is the standard grammar rule you should practice at this level. Notice that if you do use avere, the participle never changes: abbiamo dovuto.

Why is the preposition a used for a casa instead of alla?