ねばなら + ぬMust, Have to, Should

Unlike the standard ば form of hypothetical phrases, ねば makes use of the classical auxiliary verb ず in its ね form. However, the meaning is the same as the modern なければ 'if not (A)'. ねば connects to verbs in the same way as ない, and will then be followed by the negative form of なる 'to become', ならない.

Like なければならない, ねばならない will translate to 'must do (A)', or 'have to do (A)', and is simply a double negative that literally means 'if not (A), it won't be (B)'. This is a formal structure, and will mostly be found in literature.

As ね is a form of the literary auxiliary verb ず, the base of する will change to せねば, in the same way it changes to せず in other grammar patterns.

Caution

Sometimes ならない will be replaced with ならぬ. This has exactly the same meaning and just makes use of the ぬ attributive form of the same auxiliary verb, ず.

While not changing the meaning at all, the addition of ぬ will make the overall expression sound even more old-fashioned.

Fun-fact

As with other 'must' double-negative structures, the second part of the ねばならない may sometimes be omitted, resulting in ねば or せねば appearing by itself. This is similar to the modern ないと appearing without いけない following it.