Hi! This is a great question. I will cover just a few basic meanings, but it actually could be a great dissertation topic: за is used when you talk about the purpose, you need the Instrumental case идти за хлебом (instr) - go for bread посылать за врачом (instr) - send for a doctor для is used when you talk about a receiver, you need the Genitive case идти за хлебом для моего друга (gen.) - go for bread for my friend ("friend" is a receiver) посылать за врачом для моего сына (gen.) send for a doctor for my son ("son" is a receiver) Regarding you previous question about "books for children", this is the same idea and the correct phrase is "книги для детей". You also use за when talk about a cause, you need the Accusative case сидеть в тюрьме за убийство (acc.) - to serve time in jail for a murder пострадать за веру (acc.) - to pay the price for defending religious freedom уважать за возраст (acc.) - to respect for age One more case - the means: покупать за деньги (acc) - buy for money. Here "деньги" is the means, not a receiver. за счёт средств федерального бюджета - for federal budget resources A different English preposition, but the same idea ("hold" someone "using" his/her body part): взять за руку - take by the hand держать за горло - have by the throat You also use за to talk about a time frame, you need the Accusative case: выучить русский язык за год (acc.) - to learn Russian in one year за минуту (acc.) до подачи - one minute before serving Note that there is also a locative meaning of the preposition за, and a lot of other metaphorical meanings too. Hope I did not confuse you too much.