"They do not know him."
Translation:Nie znają go.
There is difference in accent. Generally, "jego" is an accented version of "go" and usually unaccented version is used but you could say: "Oni znają nie jego, ale ciebie" (They know not him, but you). You should also use "jego" at the beginning of the sentrence.
No, it would need to use the emphatic form "Jego", but even then it's pretty unusual. You can't start a sentence with "go".
Why not 'Jego nie znają' or is that placing too much emphasis on 'him' as if they know everybody else? Valid i think, but perhaps only in that context.