Learn Italian: All'improvviso, il branco si è messo a ruggire.

While both translate to 'started', mettersi a followed by an infinitive specifically implies a sudden, spontaneous, or unexpected start to an action. It pairs perfectly with all'improvviso (suddenly) to emphasize that the pack's roaring happened all at once and without warning.

Why does mettersi a use essere as its auxiliary verb here (si è messo)?

Because mettersi is a reflexive verb. In compound tenses like the passato prossimo, all reflexive verbs must use essere as their auxiliary verb. This also means the past participle (messo) must agree in gender and number with the subject.

Since a pack (branco) is made up of many animals, why is the verb singular (si è messo) instead of plural (si sono messi)?

In Italian, collective nouns like il branco (the pack), la squadra (the team), or la gente (people) are grammatically singular. Even though they refer to a group, the verb and past participle must be singular to agree with the grammatical number of the noun. Since branco is masculine singular, we use si è messo.

Is the preposition a mandatory when using mettersi with another verb?