"Can you help us tomorrow?"
Translation:¿Nos ayudas mañana?
There are rules for the position of the object pronoun. In your answer, the nos has to go immediately before the conjugated verb or attached to the end of the infinitive. Either is correct: ¿Nos puedes ayudar mañana? or ¿Puedes ayudarnos mañana?
Whether Dúo will accept either one I don't know. It seems that they are trying to teach us that sometimes "can" can be built into the main verb.
I scratched my head on this one, too. As usual this came out of the blue...
There obviously is a lesson and trial to 'imprint' a new structure. I don't know right now the name of it. But it is the structure of 'Compliance' , A sort of polite way to ask, and it has not much to do with 'poder', to be able to, to can &c .
In English a conditional form is often on those places : " Will you, would you , won't you, could you...please!
So if you try to translate word for word, it goes wrong.