Learn Italian: Il rinoceronte si è fermato bruscamente.

In Italian, the base verb fermare is transitive, meaning to stop something else (like stopping a car or a person). When a subject stops itself or comes to a halt, Italian uses the reflexive form fermarsi. Because all reflexive verbs require the auxiliary essere in compound tenses, it becomes si è fermato.

This lesson is about sudden starts (using mettersi a). Why is this sentence here?

This sentence provides a thematic contrast to the lesson's main grammar point. While mettersi a + infinitive describes the sudden inception of an action, this sentence describes a sudden stop. The suddenness here is expressed lexically through the adverb bruscamente (abruptly) rather than through a specific grammatical periphrasis.

How is the adverb bruscamente formed?

Like most Italian adverbs ending in -mente (the equivalent of the English suffix -ly), it is formed by taking the feminine singular form of the adjective and adding the suffix. Here, the masculine adjective brusco (abrupt or sharp) becomes the feminine brusca, and adding the suffix gives us bruscamente.

What would happen to the verb if we were talking about two rhinoceroses?