Translation:Many Esperanto speakers are nice and interesting.
No, you actually mustn't use it here: it changes the meaning. Here afabla is a predicative: it describes the condition of other things. If you use the accusative case here, afablan becomes a normal adjective to komunumon. This changes the meaning entirely:
Mi trovas la Esperantan komunumon tre afabla = I find the Esperanto community very nice. [afabla describes the state of the opinion I have]
Mi trovas la Esperantan komunumon tre afablan = I find the very nice Esperanto community*. [as in, I have located it]
Here are some more such examples:
Mi farbas la domon blua = I paint the house blue. [after painting it, it is blue]
Mi farbas la domon bluan = I paint the blue house [before painting it, it was blue, but no information is given about the current color]
Sometimes it can become actually ambiguous:
Mi hejmen veturigis la knabon ebria = I drove the guy home drunk.
This can mean that when I arrived at his house, the guy was still drunk, or that I was still drunk!
If you change the accusative case:
Mi hejmen veturigis la knabon ebrian = I drove the drunk guy home.
I hope this clears it up a bit!
Wow that makes sense indeed. I was a bit confused at first because I thought there would be a rule similar to French ("attribut du complément d'objet" -> attribute of the direct object). Being a French native speaker is both a gift and a burden when learning Esperanto! : )
Nice thought, but it's not. Mojosa is the adjective form of mojoso, which is derived from Moderna Juna Stilo: Taking ‘MJS’ as abbreviation of these three words and pronouncing the letters yields mo jo so, which makes mojoso, meaning something like ‘coolness’. Thus mojosa means ‘cool’.