"My sister is buying a green backpack."

Translation: Moja siostra kupuje zielony plecak.

[smartpupa]

Why not "...... kupuje zielonego plecaka" ???

[immery]

you need accusative after kupować, "plecak" is not animated so it has accusative=nominative plecak.
Then you need green to match "plecak" so it is also masculine accusative=nominative zielony.

*If you ever heard poles say it different that's because some Poles are confused with the "animation" and some other Poles use it "ironically".