"Please be careful!"
Translation:Bonvolu esti atenta!
Because ‘bonvolu’ is already in the imperative; it would be weird to have two verbs in the imperative in the same clause.
To make it more concrete, you can loosely translate ‘bonvoli’ with ‘to be so good’, then the sentence becomes ‘be so good to be careful’.
For a more rigorous definition see http://vortaro.net/#bonvoli (log in using Lernu or so).
Ĉu bonvolu = bona+volu? If so why do you translate it as 'to be so good'? Isn't it bona+estu?
Just like constructions as scivola, which means ‘curious’ (in the sense of ‘intrigued’), bonvola means something like ‘good willing’ (just like scivola kind of is ‘knowledge willing’, i.e., wanting to know = curious). Then the usual contraction of an adjective with esti is used to get to bonvoli. (Just like esti bona becomes boni.) So bonvoli sort of means ‘to be good willing’. Then if you use the imperative bonvolu plus infinitive (say, fari), you basically want that someone have a good will (= be good willing) to ⟨infinitive⟩ (say, do something). This is just like ‘please’. Hence why bonvolu can be translated to ‘please’.
I hope that makes sense!
I don't think so, as atenti is more to pay attention or to be observing/attentive rather than to be careful.
Scratch that, it works just fine as you can turn any adjective into a verb.