"I want to tell you something very important."
Translation:Chcę ci powiedzieć coś bardzo ważnego.
Accusative – genitive would be „czegoś bardzo ważnego”(cf. „Słucham czegoś bardzo ważnego”); It also changes to genitive in negations, as would be the rule:
- „Nie powiem ci czegoś bardzo ważnego”(through that's a bit stilted example, I would rather say „Nie powiem ci niczego ważnego” normally, even if it's not the same meaning 100%)
Good point, but if it's Accusative, then it's also not really neuter (that would be "ważne") but masculine (for whatever logic). The most important thing is still "it ends with -ego" :)
Generally "coś" is neuter, you can say "jakieś dziwne zielone coś" (some weird green something).
"to tell someone something" needs that "someone" in Dative.
The singular Dative 'you' is "ci", its emphasized version is "tobie". "ciebie" is not Dative.
See the table of all forms: https://en.wiktionary.org/wiki/Appendix:Polish_pronouns#Second-person_pronouns