"The parrots sing sweetly."
Translation:Psittaci suaviter cantant.
Psittacos is plural accusative, that is, it's the object of a verb, whereas psittaci is plural nominative, or the subject of the sentence which performs the action.
Thus: psittaci pulsant = the parrots hit vs Psittacos pulsant = they hit the parrots
Since you know, thus is latin duolingo and it's all about the parrot wallopin'
Hope this helps