"If we do not do sports, we will get tired."
Translation:Noi dacă nu facem sport, noi vom obosi.
In fact this would be the normal way of saying it. The form used by Duo (noi dacă nu facem...") is used rarely, when you want to emphasize the group (noi). Also do not forget that in Romanian the person can be omitted as it is already indicated by the verb. So the usual phrase would be "dacă nu facem sport vom obosi"