"He sees us."
Translation:On widzi nas.
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.
"On nas widzi" and "on widzi nas" is both correct? Is there any difference in meaning when changing the word order here? What is preferred?
Maybe it's a bit oversimplifying, but the new important information is usually at the end of the sentence, so "On nas widzi" focuses on SEES, and "On widzi nas" focuses on US.
I myself would use "On nas widzi" as the most natural word order, at least without a context.
I have a doubt on the placing of these pronouns: why an when are them placed before or after the noun?
I think a more neutral sentence would be "On nas widzi", it would just state a simple fact.
Putting 'nas' at the end gives additional emphasis on US.