"Come mai?" is a very often heard and important for you to learn idiomatic expression in Italian.
For English speakers learning Italian it is literally translated or seems to say "how never" which is not want it means.

(idiomatic) Come mai? means Why?, How come?, Why on earth?
Come mai non sei venuto alla festa? - Why didn't you come to the party? How come you didn't come to the party? Why on earth didn't
you come to the party.

Other examples:
Come mai non andiamo al cinema? ‘How come we don’t go to the movies?’

Dopo di vivere tanto tempo in Italia come mai non hai imparato la lengua Italiana?

Italia come mai non hai imparato la lengua Italiana?
After living so long in Italy how come(why in the hell)didn't you learn the Italian language?

Come mai non hai imparato bene la grammatica italiana? How come you haven't learned well Italian grammar?

Ah... mi licenzia per incompetenza..! Potrei chiederle, signore, come mai ci ha messo vent’anni per accorgersene....?”

“Firing me for incompetence! I might ask, sir, how come it took 20 years for you to notice?”

Anothe example that American use all the time is :
Non vedo l' ora ( I do not see the hour ) - Meaning I am looking forward

Another example that Americans use all the time is :
Non vedo l' ora ( I do not see the hour ) - Meaning I am looking forward to

Molto bene. Yes, this is another must know phrase or
expression.

The list is endless of course as Italians use descriptive words to describe ordinary things.

For example, sta facendo il pappagallo would be translated as making like a parrot, although it means to repeat something parrot fashion.

Che ti prende? What's the matter with you? (What takes you)
Fare un capello in quattro: To split hairs (to make a hair in four)
Togliti dai piedi: Get out of the way (remove yourself from my feet)

Paolo - Thanks so much. This kind of information is so helpful to those of us who are neophytes at speaking Italian. Would you help us even more and give us an example of a sentence using that expression????

Paolo - Thanks so much. This kind of information is so helpful to those of us who are neophytes at speaking Italian. Would you help us even more and give us an example of a sentence using that expression????

Non vedo l'ora di andare in vacanza
Non vedo l'ora di vederti
Non vedo l'ora per andare a mangiare
Non vedo l'ora di poter chiudere questa facenda

"Non vedo l'ora" possibly translates better as "I cant wait until.."

"Non vedo l'ora" possibly translates better as "I can't wait until.."

Non vedo l'ora di andare a Roma. I can't wait to go to Rome.

I'm looking forward to going to Rome. I can't wait until I go to Rome.