present + da

To describe an action that began in the past but is still ongoing in the present, English uses "have been" plus a verb ending in -ing. For example...

The line-up is really long. We've been waiting for hours.

I've been studying Italian for three months.

But Italian just uses the plain old present tense + da (+ a time expression). For example...

Aspettiamo da ore.

Studio l'italiano da tre mesi.

If you're referring to an action that took place over a period of time in the past but is now completed (i.e. no longer in porgress) you use the passato prossimo + per. For example...

Ho studiato l'italiano per 3 anni. ("I studied Italian for 3 years" - with the implication that you're not studying it anymore right now.)

And that's it! You've probably been wanting to learn this for a while...and now you have!