When does one use preposition in french (á, á la, aux, au)

à: to (generic, or specific place)

à la: to the (going to a feminine location)

au: to the (going to a masculine location)

aux: to the (in plural form)

Explanation:

These are all versions of the English words "to"/"at", but they vary depending on the gender and the number.

First, you wrote "á", that's with an acute accent.  The only letter with an acute accent in French is "e" which becomes "é".

The letter "a" only comes with grave "à" and circumflex "â" accents, so the correct to write it is "à".

à: going somewhere, usually a city, like "Aller à Paris" ("Going to Paris").

à la: going to a feminine location...  "Aller à la fête" ("Going to the party")

au: going to a masculine location... "Aller au restaurant" ("Going to the restaurant")

aux: going to many places, or a plural name... "Aller aux bars de cette rue" ("Going to the bars of that street") or "Aller aux Bahamas" ("Going to the Bahamas")