Learn Italian: Sta per piovere. - It is about to rain.

The combination of the verb stare + the preposition per + an infinitive verb creates the meaning "to be about to" do something. Here, sta is the "he/she/it" form of stare, so sta per means "it is about to".

Where is the word for "it" in sta per piovere?

Italian doesn't use a dummy subject pronoun like "it" for weather. While in English we say "It rains", Italian just says "Rains" (Piove). So, for "It is about to rain", we just start directly with the "he/she/it" verb form: sta.

Why is piovere in its full dictionary (infinitive) form?

In the stare per structure, only the first verb (stare) gets conjugated to match the subject. The main action verb always stays in the infinitive. Also, as a general rule in Italian, verbs that come directly after a preposition like per are always left in the infinitive.

Could I use this exact structure to say "It was about to rain"?

Yes! You just need to change stare to the past continuous (imperfect) tense. "It was about to rain" becomes stava per piovere. The per piovere part stays exactly the same.