* 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.

The main difference is between transitive and intransitive verbs. All the transitive verbs use avere (transitive verbs must answer to the question who, what: to know is transitive, I know someone/something; to go is intransitive, I go TO Paris, or I stay AT home). Most of the intransitive use essere (essere, stare, rimanere, andare, venire, partire, nascere, morire, diventare, piacere, sembrare...) All the reflexive verbs use essere (mi sono vestito) I know it's complicated, but this is only a general review

In English/Spanish you always use one auxiliary, to have, but in Italian thinks are trickier. But, hopefully, I am going to make it easy, although my answer is so long, it did not fit in 2000 characters. Look at this complexity as a mean to provide a bit of variation and embellishment to the language.