Puno Čega?

Say puno (or mnogo, malo, nekoliko) and the noun behind it drops into the genitive. Not the accusative you'd expect after imam or jedem — the genitive wins, because the quantity word is what's really governing the phrase.

Then a second decision follows immediately, and it's the one this exercise is really about: singular or plural genitive? It depends on whether the thing can be counted.

Sugar, water and money come as a mass, so they stay singular. Books and shoes come in units, so they go plural. And feminine plurals in the genitive famously end in -a — which makes knjiga look like the nominative singular even though it isn't.

Two to memorise on sight: novac → novca (the a falls out) and psi → pasa, which does whatever it likes.