Learn Italian: I bambini si sono spaventati per il rumore. - The children got scared because of the noise.

The verb is spaventarsi (to get scared), which is a reflexive verb. In Italian, all reflexive verbs must use the helper verb essere (to be) to form the past tense. You can think of it as saying the children "frightened themselves".

Why does the main verb end in -i (spaventati)?

Whenever you use essere as your helper verb, the past participle (the main action word) has to match the subject. Because i bambini is a masculine plural subject, the ending of the verb changes from -o to -i.

Where exactly does the reflexive pronoun si go in a past tense sentence?

It goes right before the helper verb! In this sentence, the pronoun si (themselves) is placed immediately before sono (are), rather than attaching to the main action word.

Why does the sentence use per for "because of"?

While you probably first learned per as "for", it is also the standard word used to express a cause or reason. In contexts like this, it naturally translates to "because of" or "due to" the noise.