Learn Croatian: Nisam stavio prljavo rublje u perilicu. - I didn't put the dirty laundry in the washing machine.

The verb staviti (to put) involves motion towards a destination. When the preposition u indicates direction or motion ("into"), it requires the accusative case (perilicu). If you used the locative case (perilici), it would imply a static location, meaning the action of putting something took place entirely while you were already inside the washing machine!

Is rublje a plural noun? Why does it take the neuter adjective prljavo?

In Croatian, rublje is grammatically a neuter singular noun, even though it translates to a mass or collective concept in English (laundry or underwear). Because it acts as a singular neuter noun, the adjective must match it with the neuter accusative singular ending -o (prljavo).

Since the sentence is negative, shouldn't the verb be imperfective (stavljao) instead of perfective (stavio)?

Not necessarily. While Croatian often uses imperfective verbs in negative sentences to show an action generally didn't take place, the perfective (stavio) is used here because you are denying one specific, expected event. You are stating that you failed to complete the single task of putting this specific pile of laundry into the machine.

I often hear people say veš-mašina instead of perilica. Which should I use?