Why the infinitive here, but not in the other examples?
Both forms are acceptable and have the same meaning.
"I will not make juice" is also future tense but not imperfective aspect. How does one say it in Polish?
Nie zrobię soku.
Is the pronounciation of the sentence correct? The word robić sounds to me as if there was a 'c' instead of a 'ć'.
Yeah, it sounds like that. It's hard to say a Ć immediately followed by a S, without slowing down to pronounce them separately