The recordings of this sound in DuoLingo are not good. In Portuguese, when you have two r's together, like correr, the rr becomes an h sound, more like the German ach than the English h. DL makes it sound more like the English h. I'm having trouble with the rr sound too. I first learned the trilled r in Spanish, so I had to learn how to do the back-of-the-throat Portuguese sound in place of the Spanish r and rr.
Hi Dentaurthur. For doubts about verb tenses, I consult http://www.verbix.com/
For example, the command or imperative form for "correr" is:
SINGULAR - tu corre (used mainly in Portugal) - ele, ela, você corra - nós corramos
PLURAL - eles, elas, vocês corram
I hope this helped. Bom estudo.
well, "Vós" is different, you can safely ignore that. It's practically not used anymore, both in Brazil and in Portugal (and all the other countries where Portuguese is spoken). There are still rare uses of it in some northern Portuguese regions but that's it. I don't think you will ever find a sentence with "vós" here in Duolingo.
"Tu", on the other hand, is not only extensively used in Portugal but also in parts of southern Brazil, as people already said in other comments.