Modals and Reflexives: Auxiliary Shift
When using a modal verb (dovere, potere, volere) in the past tense with a reflexive verb, you have a choice about where to place the reflexive pronoun (like mi, ti, or si).
If you place the reflexive pronoun before the modal verb, you must use essere as your auxiliary. Because you are using essere, the past participle of the modal verb must agree in gender and number with the subject.
| I had to step away hastily due to a sudden illness. | Mi sono dovuto assentare precipitosamente per un malore. |
If the subject is feminine, the past participle of the modal verb changes to match. Watch how dovuto becomes dovuta for a feminine subject:
| She had to vent after all the stress of the day. | Lei si è dovuta sfogare dopo tutto lo stress della giornata. |
The same rule applies to plural subjects. The auxiliary is still essere, and the modal's past participle becomes plural:
| The traffic was terrible, so we had to stop. | Il traffico era terribile, perciò ci siamo dovuti fermare. |
This shift to essere happens with all modal verbs when the pronoun is placed at the front. Here is an example using volere, where the participle becomes voluti to match a plural subject:
| They didn't want to justify themselves for an unfounded accusation. | Non si sono voluti giustificare per un'accusa infondata. |