ねばならない

ねばならない means "must": the speaker or writer sees a duty, a necessity or a resolve. It replaces the ない of the ない-form, so 行かない gives いか行かねばならない and 食べない gives たべ食べねばならない. It belongs to writing, speeches and formal statements.

する takes the old stem せ, not し.

• × 解決しねばならない → 解決せねばならない

ねばならない or なければならない

Both mean "must". なければならない is the standard form in speech and writing, and なきゃ is its casual contraction. ねばならない sounds formal and solemn, so in conversation it comes across as stiff or theatrical.

Usage notes

These go beyond the full form ねばならない.

ねばならぬ

ねばならぬ swaps ない for the older ぬ. It is more literary still, and turns up in slogans and fiction.

ねば on its own

The sentence can stop at ねば, with ならない left unsaid. It is a resolve said to yourself, and you hear it in casual speech as a half-joking "gotta".

In the middle of a sentence, ねば is an "if not": 急がねば means 急がなければ.