How to know if we should use “за” or “для”?
Hi! In this case two different prepositions are used because they mean two different things.
Для is used when an object or also something abstract is directed to somebody. It is used with the Genitive case.
Подарок для папы. A present for Dad.
Компьютер для Сергея. A computer for Sergey.
За is used with the Instrumenal case here. When you use this preposition with verbs of motion, it means "to come somewhere to take something".
Я пришёл за тобой.
I came for you, I came to take you with me.
Мы зашли в магазин за хлебом.
We entered the shop to buy some bread.