- I will tell mum.
- Don't tell her!
If somebody want to translate it literally, word by word, applying basic grammar rules like the imperative or the future tense (but without declination)...
- Ja powiem mama.
- Nie powiedz jej!
A pole would understand this. But not this:
Będę mamusiu powiedział.
Nie niej powiedź!
1. Why mamusiu? Why diminutive? And why the past tense if in the original text there is a future tense? Ok, maybe you wanted to use "będę powiedział", which is a form that is theoretically possible to create :) Of course, if you don't take into account that "będę powiedział" is a strange mixture, verbs like "powiedział", "zrobił" do not make future form in such a way. It works for "mówił", "robił" - for these which indicate a repeated or continuous activity, like continuous tenses in English. And why this "mother" went into the middle? Aha, it was supposed to look like German :) Ich wird der Mutti sagen :)
2. But why here "powiedz" at the end? It's neither English, nor Polish, nor German :)
[EN] Don't tell her!
[PL] Nie mów jej! -> Not speak her. (speak because powiedzieć is here replaced by mówić - the speaker wants the mather not tell this not in a one moment in time, but at all)
[DE] Sag ihr nicht! -> Tell her not.
Nowhere the verb "to tell", "mówić", "powiedzieć" goes to the end :)
"Zaraz wracam!" = I'll be right back! (and correct) cf. English: "I am coming right back.", carrying the idea of a FUTURE action!
"Zaraz wrócę!" = I'll be right back! (and incorrect Polish) cf. (ungrammatical as well as illogial EnglishLOL) "I come right back.." [as I do every day].
As for me, both look well, even if you don't do this ever day.