〜ねばならない・〜ねばならぬ
must, have to (formal)
Verb ない-stem + ねばならない / ねばならぬ (する → せねばならない)
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ねばならない is a formal, literary "must": 約束は守らねばならない, "promises must be kept".
It's built from the classical negative ぬ in its conditional form (ね + ば), so it's the old version of なければならない (N5). ねばならぬ is even more old-fashioned and resolute.
It attaches to the ない-stem: 守る → 守ら + ねば, 進む → 進ま + ねば, and する → せねば. Careful: する gives せねば, not しねば.
You'll meet it in speeches, novels and the speech of dignified characters. The short form せねば (without ならない) is a common spoken resolve: 急がねば!, "I must hurry!".
Examples
約束は守らねばならない。
Promises must be kept.
我々は前に進まねばならない。
We must move forward.
明日までに報告書を書かねばならない。
I have to write the report by tomorrow.
そろそろ出発せねばならない。
It's about time we set off.
真実を明らかにせねばならぬ。
The truth must be brought to light.