Alguno vs. Un / Una
When would you use Alguno instead of un / una to mean "a"? Thanks in advance 
3 Answers
Here is a helpful link: Unos vs. Algunos
By the way, I highly recommend reading the articles on Spanish Grammar at Spanish.About.Com. I like how when you read one thing you want to know about this website always makes suggestions about other topics that are related to what you just read. There are also links within articles that will explain in more depth the topic that is being discussed.
Spanish Grammar Topics at Spanish.About.Com
I often see alguna translated as "any" which is somewhat different from one in English. Any basically means "one or more than one." But in English a can mean one or any depending on context.
In Spanish una can mean one of a feminine object instead of the English "a" which also often means "any" in context. For example, if you asked someone "Do you have a pen" you are not really trying to say "Do you have ONE pen?" You are really asking them if they have ANY (one or more than one, but you just want one) pens that you might borrow.
So ¿Tienes una pluma? Could mean "do you have one pen"?
¿Tienes alguna pluma? Do you have any pens? (note that English plurals are often singular in Spanish)
Which could also be stated as "Do you have a pen" in English with no change in meaning. However, we would almost never say "Do you have one pen"
So, in summary, alguno can be used to mean "any" and in English "a" often means "any" in contect, so, yes, I would say that alguno can definitely mean "a" and it can be used in Spanish to avoid the possibility of una mean one instead of a/an.
I should point out that the above explanation is probably more theoretical than practical. While una can mean either one or a/an, it is normally understood from the context which is meant, and Spanish speakers use una much more often than ninguna to mean "a/an" The use of ningun/o/a seems to be used most frequent as "any" as in "do you have any questions" ¿Tiene alguna pregunta?
Good question, and yes algún/o/a can* mean "a". I'm not fully qualified to answer this question completely, but I'll give you a couple examples. It tends to change the feeling slightly more than the over all meaning. P.S.These aren't direct translations...
¿Tienes una pluma? --- Do you have a pen? ¿Tienes alguna pluma? --- Do you happen to have a pen?
¿Tienes una pregunta? --- Do you have a (specific) question? ¿Tienes alguna pregunta? --- Do you any questions?
Keep studying/researching and listen to native spanish speakers. Also watch what LaHojaDeArce was talking about - Lesson 2.11
'Alguno' is not going to mean 'a'.
Una mujer me dijo que fue un hombre detrás de ella....
Algún día algunos de mis amigos van a ir a mi casa.....
Alguno de ellos (one of them)....
Algunos de ellos (some of them) .....
