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’.
And four years later "agreeable" isn't accepted. It must be a US/UK thing but here in UK nice in this context means quaint, or even facetious as in smarmy and insincere. "Nice people" isn't something I'd want to be. To say "that's nice!" is usually ironic and means "it's disgusting".