"They do not know him."
Translation:Nie znają go.
The genitive and accusative forms are identical, so "go" stays unchanged, no matter whether the verb is negated or not.
"Jego" is an emphatic form which is totally correct, but very unlikely in this context. It's like saying: No, it's HIM that we don't know, not the other guy.
There are similar sentences where "jego" is accepted, but I'm removing those answers wherever I see them. See this comment for additional information: https://forum.duolingo.com/comment/12749660?comment_id=37924936
1st and 2nd person forms of the verb always give you enough information by themselves, you don't need the pronoun. 3rd person forms need the subject to be known before - whether it's "Mark", "Joanne" or "My parents". But in reality the subject is usually known, it's the Duolingo sentences that do not have it.
I am confused about your company example... but if I was talking with my colleagues about our company, then I guess it's 3rd person singular, "The company does something".
Hi Thanks your comments. Re Ltd company in UK it's a legal entity, has similar rights, obligations as an individual. Just trying to gain some context in actually using the language in a commercial setting. I enjoy the App by and large but feel could be improved by having easily available tables re Pronouns etc as I'm spending a lot of time screen shooting Clozemaster and other new Polish language sites popping up now. I understand you, or the company is covering a lot of ground in terms of languages and I guess it all supply and demand. At the moment I'm devoting all my 'language time' just to Polish which may have fewer subscribers than other languages.