also: でなければいけない · でなければならぬ
must be; have to be; ought to be
Used after a noun or na-adjective stem with で to state a required identity, status, quality, or condition. It is the copular counterpart of なければならない: literally, 'if it is not ..., it will not do.'
A leader must always be calm.
Applicants must be at least eighteen years old.
A leader must be fair.
Formed from copular で plus なければ, the conditional of ない, plus ならない, literally 'if it is not ..., it will not do.' The variant ならぬ uses an older or more literary negative form of なる.