Learn Croatian: Pospremi igračke prije večere! - Tidy up the toys before dinner!

They actually belong to two different cases! Igračke (toys) is the direct object of the verb, so it is in the accusative plural. Večere (dinner) follows the preposition prije (before), which always forces the noun into the genitive case. For feminine nouns that end in -a, both the accusative plural and the genitive singular happen to end in -e.

How does pospremi differ from the similar verb spremi?

While both are perfective verbs, the prefix po- changes the focus. Spremiti generally means to put something away, to store it, or to prepare. Pospremiti specifically means to tidy up or clean a physical space, like a room, or an unruly group of items, like toys.

Could I use the imperfective verb pospremaj for this command instead?

You could, but it changes the expectation. Using the perfective pospremi means you want the task completely finished before dinner. If you used the imperfective pospremaj, it would sound more like 'spend some time tidying up,' focusing on the activity itself rather than getting the job entirely done.

How do you build the imperative form pospremi?