I couldn't figure out the grammar rule here. I know it is possessive pronoun but I don't know the case (it seems nominative here)
Sie kauft die blaue Mütze für ihren Bruder.
Please tell me why it is ihren here?
German Language Stack Exchange is a bilingual question and answer site for speakers of all levels who want to share and increase their knowledge of the German language. It's 100% free, no registration required.
Sign up to join this communityShort answer:
für asks for the accusative case, Bruder is singular masculine, so one has to use ihren there, because this is the accusative masculine singular inflection of ihr.
Additional comments:
In general, you should be able to determine the case by searching for the preposition in some dictionary or even on Google.
Actually, knowing the case taken by a preposition is is not complicated at all if you have a good memory (or have practiced enough).
By the way, this wikibook has a decent lesson about prepositions and cases.