Learn Italian: La strada si è allagata a causa della pioggia e della grandine. - The street flooded due to the rain and the hail.

In Italian, the active verb allagare needs a direct object (for example, if a broken pipe flooded a room). When the subject itself is the thing filling with water, you use the reflexive form allagarsi. Because all reflexive verbs take essere in the past tense, the participle must also agree with the feminine singular subject, giving us si è allagata.

This lesson is about weather verbs. If I used verbs instead of the nouns pioggia and grandine, which auxiliary verb would I use for 'it rained' and 'it hailed'?

Weather verbs like piovere (to rain) and grandinare (to hail) are very flexible! In modern Italian, you can use either avere or essere in the past tense. This means ha piovuto and è piovuto are both totally correct, just like ha grandinato and è grandinato.

Why do we repeat della before grandine? Couldn't we just say a causa della pioggia e grandine?

In English we often share one 'the' across a list (the rain and hail). However, Italian strongly prefers to repeat the combined preposition and article (like della) for each noun. It sounds much more natural and grammatically complete to an Italian ear to say della pioggia e della grandine.