Для and за
Both are translated as "for". What is the difference and how do I know which one to use?
Для is a possessive, you must write it when something belongs to someone, for example, "This present is for Lena"=Этот подарок для Лены". За is used when you're talking about time, for example, "за три недели"=for three weeks, during three weeks. Also, За can mean "behind", for example: "За домом"=Behind the house. За is used in other cases too, but you will learn them eventually. :)
- Для + genitive = for as in "this is for you", to as in "to do this, ..."
- За + accusative = for as in "do something for (instead of) someone", for as in "be punished for doing something", also in "выйти за..." = marry ... (used only for women, for men it is "жениться на + prepositional)
- За + instrumental = behind, also as in "за хлебом" = to buy bread
I'm sure I missed something %) All in all, you should remember that "для" always requires genitive, and "за" is used both with accusative and instrumental.