ずには済まない / ないでは済まない / ずにはすまない【ずにはすまない / ないではすまない / ずにはすまない】

ずには済まない and ないでは済まない mean "won't get away without doing": after what has happened, other people or the situation will demand the action. ないでは済まない goes on the ない-form as it is; for ずには済まない, replace ない with ずには済まない, and する becomes せずには済まない. The two mean the same, and ず is the more written of the pair. 済まない is often written in kana, すまない.

• × しずには済まない → せずには済まない

すむ済む is "be settled", so the literal sense is "it won't be settled without ~". The pressure comes from outside – duty, other people, what the situation requires – and the first half often gives the reason, with いじょう以上, のだから or からには. The sentence often ends in だろう.

The verb can also be something that will simply happen to someone, as the result of what they did.

With the passive, ずにはすまされない済まされない is stronger: nobody will allow you to leave it undone.

ずには済まない or ずにはいられない

ずにはいられない is an urge from inside you – you can't hold back a laugh or a worry. ずには済まない is pressure from outside.

ないでは済まない or だけでは済まない

After a noun, or a verb with だけ, ではすまない済まない says something won't stop at that level: it will take more. With the ない-form, it says the action itself can't be skipped.

Both forms are formal. In conversation the same idea is 〜～ないわけにはいかない or 〜～なきゃいけないだろう.