All three accusative prepositions für, gegen, and um can mean "for" in certain situations. Can anybody give examples when to use which of the three?
There is no rule when to use which one, especially in case of
um. You must simply learn which verb demands one of these three words. Some verbs and phrases use
um and there is no reason why:
Um Gottes willen! um einen Gefallen bitten
In my feel for language I tend to think
um in the meaning of this topic here is a relict of former times and only used rarely in certain fixed phrases, but I may be wrong.
gegen are used for all other stuff where
um is not used. I cannot find an example where
um can be replaced by one of them.
Gegen is used when there is an exchange of things, commonly something for the price of another, and it shows the two directions of exchanging, for example
Ware gegen Geld.
Für in the meaning of "for" is commonly used to tell the addressed receiver ("Für wen?") of the result, for example
für alle, or the cost itself ("Für was?"). But usually you can use
für also as replacement of
gegen, it more underlines the price in contrast to
gegen which underlines the exchange, but not inevitably. For example
Ich mach das nur für Geld accents a bit more what the price is ("Geld"), and
Ich mach das nur gegen Geld accents a bit more that there is actually a price and that it's not for free.