"The dog catches a fly."
Translation:La hundo kaptas muŝon.
Every time someone DOES SOMETHING to something else in Esperanto, assume that the 'something else' is in the accusative. The dog (someone) catches (does something to) the fly (something else). The dog (subject of sentence) catches the fly (object of sentence) so the fly 'n'eeds an 'n'. There are exceptions!!!