Learn Italian: Piove da ieri. - It has been raining since yesterday.

In Italian, when an action started in the past and is still happening right now, you use the present tense along with the preposition da. English uses the present perfect continuous (has been raining), but Italian simply uses the present tense!

Does da mean since or for in this sentence?

In this specific sentence, it translates to since because ieri (yesterday) is a specific starting point in time. However, da is also used to mean for when talking about an ongoing length of time, like da due giorni (for two days).

Could I use per instead of da here?

Not if you want to say it is still raining! The preposition per is used for completed durations. If you used per, you would also need a past tense verb, like ha piovuto per tutto ieri (it rained for all of yesterday), which means the rain has entirely stopped.

Why is there no word for it before piove?

In Italian, you do not use subject pronouns with weather verbs. The verb piovere (to rain) just takes the third-person singular form piove, and the word it is automatically understood.