Can I have a beer?
Actually, neither is truly correct. In English the proper, but not common, request is "may I have a beer?" or "may I have two beers"? I don't think there is an equivalent in Spanish.
Actually, now that I think about it, as qfreed pointed out, there is an equivalent to "may I have" in Spanish by using the subjunctive but it needn't follow "qué":
"Pudieras darme una cerveza, por favor?"
Roughly (and sort of idiomatically), "could you kindly give me a beer please."
Can and May and Could
Can is related to ability (habilidad)
May is related to permission. (permiso)
But we interchange them all the time.
"Could I have a beer?" is also used and for some reason considered more polite than can.
The more correct and polite word to use when requesting something is "may."
However, you hear "can" and "may" used interchangeably in everyday speech. So much for politeness...
Some smart a@# may respond, "I don't know, can you?" Technically, "can" means "able to", where "may" means "allowed to". In day-to-day conversation, most people use "can", even if it isn't technically correct.
Depends who you're asking. Are you in the local bar or in someone's home? If you're in the bar, are you asking the bartender/waiter/waitress or a person you'r with.
In every case it would be good to say "please" either before or after - "please can I have a beer (or two)" most likely addressed to your friend or "can I have a beer please" addressed to the bartender etc.
As always, the best solution is to listen to what other folk say and copy it (well unless they get a black eye - lol).
...and if you want to sound a little more macho you can always say to the bartender "I'll have a beer (two beers) please".
Whereas! We colonials just say "give us a beer thanks" and keep em coming. We never put please at the end always thanks even before we receive the beer. This is in a Hotel of course .
It's fine.You could also say, "Two beers please" Pero, cuidado, van a decir "What kind of beer? "Lager" es como San Miguel etc. So you could say "Two bottles of lager please..." Y ahora van a preguntar, "Stella, etc.." preguntando como, Estrella? San Miguel? etc.