Has the use of the imperative tense morphed into use of the present subjunctive. According to my grammar books the imperative of Limpar for instance is limpa whereas the answer given by Duo as imperative, Limpe, is actually present subjunctive. Help please Paulenrique. I confess that I am struggling with the imperative.
I'll do my best to try to explain this as a intermediate learner of Portuguese and perhaps one of the native speakers can add to this...
You recall that there are different ways to say singular 'you' depending on the Portuguese-speaking region? In Portugal and in some parts of Brazil they use 'tu,' while in most parts of Brazil they use 'você.' Well the same can be said of the imperative mood. So in Portugal for the imperative of the verb 'limpar' you'd say 'limpa' but in most of Brazil you'd say 'limpe.'
You're correct that the 'você' form (limpe) is exactly like the 'eu/ele/ela/você' form of the present subjunctive. I believe this is true for pretty much every verb.
Additionally, there exists a negative imperative mood, but only for the 'tu' form ('limpes'). So if you are in Brazil and you've been using 'você' you don't have to worry. It's the same form in both positive and negative.
Lastly I'll add that you can avoid using the imperative all together. It's basically a command and can be see as kind of rude in certain social contexts. Instead of telling someone to do something, try using the conditional and asking them to do something. ex: Instead of saying "Traga-me a minha conta por favor." You can say "Você poderia me trazer a minha conta por favor?"