* Verbs indicating "modality", like to be able to ("can")=potere, to have to/must=dovere, and also to want=volere can use both essere and avere: Non ho/sono potuto venire. I couldn't come. Sono/ho dovuto venire. I had to come. Non sono/ho voluto venire. I didn't want to come. Picky grammarians would argue that essere is more appropriate here, but in spoken language both essere and avere are used. And usage makes rules, not professors! BTW, note dovere as a noun means duty, volere means "will" and potere "power". So to have to in Italian is simply to have duty, "to duty" and so on... * With impersonal verbs (they have no subject, we conjugate them at the third person - it as in English) you always use essere: è successo/avvenuto/accaduto it (has) happened except with impersonal verbs referring to weather conditions, with which you can use either essere or avere, as you wish, just like with modals: è/ha piovuto it has rained (it rained) è/ha nevicato it has snown (it snowed) Anyway you can simplify this rule and use essere with all impersonal verbs and you will be always right. With impersonal verbs, you see in English you have an "it" subject, but no subject at all in Italian, yet you conjugate at the third person as in English. * With to be=essere, you have to use essere itself, so "has been= è stato/a", you cannot mix essere and avere and say "ho stato/a" Sono già stato/a qui. I've already been here. Note if a woman is speaking, she will use stata, a man "stato". * With passive, you always use essere: Mangio la mela. I eat/am eating the apple La mela è mangiata da me. The apple is eaten by me.