Breakdown of Powiedz mu, żeby natychmiast tu wrócił!
tu
here
on
he / it (m)
powiedzieć
to say (perfective)
żeby
so that, in order to
Questions & Answers about Powiedz mu, żeby natychmiast tu wrócił!
Why is wrócił in the past tense if I'm telling him to do something right now?
In Polish, reported commands and desires with żeby require the verb that follows to be in the past-tense form (specifically, the L-participle). Even though you want him to come back now, you must say żeby + wrócił. You cannot use an infinitive here because the person receiving the command ("he") is different from the person passing it on ("you").
Why use mu instead of go or jego?
The verb powiedzieć (to tell/say) takes the dative case for the person you are speaking to. Mu is the short dative form of "he". You would use go or jego for the accusative or genitive cases, but verbs of communicating usually require the dative for the recipient.
Do I need to include on (he) after żeby?
Is the comma before żeby strictly necessary?