"He sees us."
Translation:On nas widzi.
Well, duolingo is inconsistent with accepting or not omitting on/ona/ono.
In Polish language it can be omitted and the sentence is correct.
Widzi nas. is a correct sentence
But you cannot tell if it is he ,she or it, you need context. It is implied that someone/something sees you, but not gender ( social or grammatical) of that someone / something.