"I do laundry for seventy-one boys."
Translation:Peru pro sedmdesát jedna chlapců.
"I'll do it for you" means two things:
- Udělám to pro tebe. - I'll do it for you, for your benefit, to please you, as a favour to you.
- Udělám to za tebe. - I'll do it so that you don't have to, instead of you.
You can apply these two different meanings as a guide to other uses of "for", many verbs/contexts will only allow one of them. For example "Peru za 71 chlapců" is very unlikely - as if there was a huge laundromat that employed 71 boys and today they don't have to work, I'll do the washing for them.
Sometimes it's good to learn how various verbs form bigger structures together with prepositions. E.g. buying/selling/trading/renting/getting something for a price will always use za:
- I bought these pants for my father for 200 crowns. - Koupil jsem tyto kalhoty pro mého otce za 200 korun. -- or we can use the dative instead of the first "for": Koupil jsem tyto kalhoty mému otci za 200 korun.
- We're fighting for freedom for our children. - Bojujeme za svobodu pro naše děti. (We're fighting to gain freedom for the benefit of our children.)
- Thanks for all the fish! - Díky za všechny ryby!
Notice how whenever "for" corresponds to "za" in Czech, it does not mean "for the benefit of".
Sometimes, "for" translates to something else:
- We're waiting for you. - Čekáme na tebe.
Or nothing at all:
- I've been waiting for 2 hours (already). - Čekám už 2 hodiny.