"Come mai?" is an Italian equivalent of "How come?"
The adverb "come" means "how." The adverb "mai" means "ever." The pronunciation is "KOH-meh meye."*
*The sound "eye" is similar to that in the English noun "eye."
"How is he?" in English is Come sta? in Italian.
Come sta? in Italian is "How are you doing?" in English.
"How do you do?" in English means Come stai? in Italian.
"Like me" in English is Come me in Italian.
Come va?
"Please come!" in English means Vieni per favore! in Italian.
"Hello! How are you?" in English is Ciao! Come stai? in Italian.
"How do you say?" in English is Come si dice? in Italian.
"How can I help you?" in English is Come posso aiutarvi?in Italian.
"Good! How are you?" in English is Bene! Come stai? in Italian.
"You will come back someday!" in English is Un giorno tornerai! in Italian.
"Will you come to meet me?" in English is Vieni ad incontrarmi? or Vieni a incontrarmi? in Italian.