How to say “must do”

The tone of voice for phrasing “must do” in Japanese sounds like you are saying “I am inclined to–” or “one(a person) must —” .

Versions of “must do” (if do not do, its no good)
Neg-い+きゃ ならない(/なりません) = (very casual)
Neg-い+ければ ならない(/なりません) = formal

Neg-い+きゃ いけない(/いけません)=(very casual)
Neg-い+ければ いけない(/いけません)= formal

今日の夜(/今晩)、勉強しな ければ ならないって。
今日の夜(/今晩)、勉強しな きゃ ならないって。 (more casual)
I hear (from Mary) that she has to study tonight.

Other ways to say is with  basic formVERB + べき + (です*optional)