Note the difference between sitzen and setzen. Sitzen is static, eg "Ich sitze" = I am sitting (and I am not moving). Setzen involve movement: "Ich setze mich" = I seat myself, I sit down.
In middle school .. lol we heard this a few times. Native English speaker with German class experience in middle school
Yes. "Setzen Sie" is formal imperative.
I believe you could also say "Setz(e) sich" for informal singular imperative.
that would be 'setze dich' for the informal singular, but only to be used to a friend or a (young) child as the Germans tend to be rather formal, so Setzen Sie is safer to use.
That would be Setzen Sie sich! to mean Sit down! It needs the sich to be reflexive.
Otherwise it needs some other thing as the object, eg. Setzen Sie die Tasse ab!
What does 'sich' have to do with this sentence? Can't we just say 'Setzen Sie'?
Setzen is transitive. In other words you have to 'seat' something or somebody. In this case, it's yourself.
'Sit down' is too peremptory for politeness. 'Do sit down' softens the command; the word 'do' is offered in the word bank, but unfortunately not capitalised.