ずにはすまないWon't get away without, Won't get off without, Have to, Have no choice but to, Can’t not

ずにはすまない is used to indicate that one ‘has no choice but to (A)’, or ‘won’t get away without doing (A)’. It is a formal expression that is a combination of the negative classical auxiliary verb, ず, the case-marking particle に, the adverbial particle は, and the standard negated form of the う-Verb 済(す)む ‘to come to pass’, or ‘to finish’. Due to this, the more literal translation is ‘it will not come to pass that (A) is not done’, with the more natural English interpretations conveying the same feeling.

As we can assume by this expression beginning with ず, it will exclusively follow verbs.

Just like other instances where ず is used with する, ぜず will be the correct conjugation pattern.

The standard (ない) negative form of a verb may also be followed by ではすまない. This has the same meaning as ずにはすまない.

Caution

This construction will tend to sound unnatural in cases where the speaker only considers (A) to be something that is necessary based on their personal feelings. (A) is usually something that would be the expected course of action according to anyone.