There is a difference!
Conseguir is "te be able/capable of doing".
But poder is "to have permission or to have nothing holding you back"
Eu não posso jogar porque eu devo estudar = I can't play because I must study
Eu não consigo jogar, é muito difícil = I can't play, it's too difficult.
Não consigo dormir = I can't sleep = I try, and I try, but I just don't sleep. I try, and I try, but all the things holding me back prevent me from sleeping. I can't handle it. I can't manage to (go to) sleep
Não posso dormir = I can't sleep = I'm not allowed to. If I do that, something bad might happen.
- I've got my mind full of thoughts, eu não consigo dormir because of that.
- I'll drive from here to São Paulo, eu não posso dormir. (Never "consigo" - it's not about "being able" or "manage to".)
- Não consigo acertar o alvo!!! = I can't manage to hit the target
- Não posso acertar o alvo, or else my team mate will not be qualified because I'll take his place.
You use "conseguir" when you can achieve to do something, and "poder" when you're able to do it. In this sentence you can use both, actually. But "poder" has other meanings, like have the permission to do something, so usually "conseguir" is used to this meaning. Você não pode dormir? - You can understand that someone ir asking if this person has the permission to sleep or if this person is able to sleep.
This verb is an enigma!! I have been in Brazil 5 months and it still baffles me at times. I believe it most perfectly translates in English to "manage to", I speak French and it would translate to "réussir" which means to succeed, but is used a alot in day to day things. All the feedback is helpful!
Duolingo, perhaps because it uses the same translation pairs in its English from Portuguese course, tends to reject declarative questions like "You can't sleep?" and prefers the standard inverted order "Can't you sleep?" ("Are you not able to sleep?") even though both styles translate the Portuguese question.
Indeed. Because of the shared sentences and the importance of teaching English learners the standard question word order, we decided not to accept univerted questions.
If we accept them, people will simply assume it's ok to do that all the time and that the inversion is just fancy. Since their main purpose in learning is to get better jobs, pass exams and things like that, we are trying to make their life easier :)
In the case of sentences where the auxiliary is "can," I think you should accept the uninverted word order. In many varieties of English (including my own), an inverted question with "can" implies frustration, e.g. "Can't you sleep?" would imply that I am frustrated that the person I'm talking to is not sleeping (compare more common examples like "Can't you be quiet?"). If I was genuinely asking whether or not the person could sleep, I would say, "You can't sleep?" or more colloquially (and more probably) "Can't sleep?"
Thanks for this answer, Dan. As a native American English speaker, though, I think there should be an exception for this example. The most natural and informal way I'd ask this question is "Can't sleep?" Inverting the verb to "Can't you sleep?" - to me, it almost implies that the speaker is wondering whether sleep is an option for the listener. I do totally agree that in the big picture, it's far more important to help students get the hang of verb inversion in English.