0Hmm. 'Formal' and 'informal' are not voices-- the voices are 'active' and 'passive'-- but I know what you mean, Vincent. 'Must' as a noun has been around a long time, but I suppose if I were writing a proper 'formal' sentence, I would use 'requirement'.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.