You use this to say you absolutely must do something. If you don't do it, things will not be okay.
Verb [ない-form] + ずにはすまないPrimary
You use this to say you absolutely must do something. If you don't do it, things will not be okay.
今回の不祥事については、責任者として謝罪せずにはすまないだろう。
Regarding this recent scandal, as the person in charge, I will almost certainly have to apologize.
親友の頼みだったので、たとえ困難でも引き受けずにはすまなかった。
Since it was a request from my best friend, I had no choice but to accept it, even though it was difficult.
多大なご迷惑をおかけしたのですから、直接お詫びに伺わずにはすまないと考えております。
As I have caused you a great deal of trouble, I believe I must visit you in person to apologize.
法律が改正された以上、新しい規則に従わずにはすまない。
Now that the law has been amended, we have no choice but to follow the new regulations.
必ずしも社長自らが出向かずにはすまない、というわけではない。部長でも十分だろう。
It's not necessarily the case that the company president himself must go. The department head would probably be sufficient.
This phrase shows you really must do something. It's like a strong inner voice or a rule everyone knows. If you don't do it, things will feel wrong or unfinished.
This is like 'must do'. It is not like 'cannot not do'. This grammar is more formal. It shows a strong duty. It is also different from 'without doing X, Y cannot happen'. This grammar means you must do something. The other means something needs a condition.
Don't use this when you want to say "I don't eat." This is for things you must do. You make this form from a verb. Take the "nai" form of a verb. Change "nai" to "zu." Then add "ni wa sumanai." For example, "taberu" (to eat) becomes "tabenai." Then it becomes "tabezu ni wa sumanai." The verb "suru" (to do) is special. It becomes "sezu ni wa sumanai."
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