Learn Italian: Il centro si è allagato, perciò hanno annullato i festeggiamenti. - The downtown area flooded, therefore they canceled the celebrations.

Not in this context! True weather verbs (like piovere or nevicare) are flexible and can take either auxiliary. However, allagare (to flood) isn't a strict weather verb. Here, it is used in its reflexive form, allagarsi, to show that the center got flooded. Because it is reflexive, it must take essere.

Why is there a si in si è allagato?

In Italian, allagare without the si means to flood something else (for example, a river flooding a town). When you want to say that something became flooded on its own, you use the reflexive form allagarsi. Since the downtown area is the subject that got flooded, we need the reflexive pronoun si.

What is the difference between perciò and perché?

Perciò means "therefore" or "for this reason," which introduces the result of an event. Perché means "because," which introduces the cause. Here, the flooding is the cause and the cancellation is the result, making perciò the correct choice to link the two ideas.

Who is the subject of hanno annullato?