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.'
リーダーは常に冷静でなければならない。
A leader must always be calm.
申請者は十八歳以上でなければいけない。
Applicants must be at least eighteen years old.
指導者は公正でなければならぬ。
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.