Article A x AN
We usually learn in language courses to use A before consonants (A car) and AN before vowels (AN umbrella).
In fact, the rule is to use A before consonant SOUNDS and AN before vowels SOUND. So we have:
A hotel [rou-tél]
AN hour [áu-ãr] - H mute
Both words start with H, although they are pronounced in different ways.
AN umbrella [ãm-bré-la]
A university [iú-ni-vãr-si-ti]
Same as before, both words start with U, although they are pronounced in different ways.
Because of this we say:
I bought an IBM computer. [ai-bi-em]
Are you an SAP person? [és-ei-pi]
We have an RCA television for sale. [ar-si-ei]
He's a European citizen. [yu-ro-pi-ãn]
That happened about a year ago. [y-ãr]
He got the job as a waiter. [uei-rer]