"Please give me the apple."
Translation:Bonvolu doni la pomon al mi.
The verb that goes after "bonvolu" is always in the infinitive form (the "-i" form). This is just how "bonvolu" works. (It's sort of like saying "have the good will to...".)
But "donu" isn't a bad guess. If you wanted to say "give me the apple" without saying "please", you would say "donu al mi la pomon".
Alternately, if you really want to use the imperative (command-form) of doni -- and you still want to be polite about it -- you would have to change the word for "please" to an adverb. So you could say "Bonvole donu al mi la pomon." This is a less common form but you do come across it.
Why wouldn't "Bonvolu donas al mi la pomon" work? The command implies addressing another, and the infinitive doesn't do that. For example, "please open the door" translated to Spanish is "Por favor, abre la puerta." The verb abrir is conjugated to directive. On the other hand, it could also be translated to "Favor de abrir la puerta," So at the very least both should be accepted.