"Do they eat bad fruit?"
Translation:Ĉu ili manĝas malbonajn fruktojn?
the plural form of fruit is fruits. the "plural" you are thinking of is the collective noun. Like a Pride of Lions. It is one group, of many, but it's still one group so it's singular. If the translation really needs to be plural then it should be "bad fruits". Kreilyn is correct
For example "He brought me many bad fruits" You wouldn't say "He brought me many bad fruit" That doesn't make any sense.
Does this mean are they currently eating bad fruit, or do they eat bad fruit on an ongoing or recurring basis?
I think in English if you wanted to know if they are eating spoiled or otherwise bad fruit at the moment you'd say "Are they eating bad fruit?". Certainly in the English I've heard and spoken.