ざるを得ない: circumstances leave no other choice
Express an action that external circumstances require despite one's wishes. Learn the special form of する, past completion, and the difference from an emotional impulse.
A choice imposed by the situation
予算が足りないので、計画を変えざるを得ません。 means that the budget shortage leaves no choice but to change the plan. The stance is a lack of alternatives rather than a preferred choice.
This need not be an explicit rule imposing a duty. 電車が止まり、歩かざるを得ませんでした。 says that trains stopped, leaving the speaker no choice but to walk. Transport circumstances restrict the choice.
Replace ない, but watch する
Remove ない from 歩かない and add ざるを得ない to form 歩かざるを得ない. 食べない becomes 食べざるを得ない; the standard reading is ざるをえない.
する takes せざるを得ない, not しざる. 会議を中止せざるを得ませんでした。 says that canceling the meeting was unavoidable in the past, not merely that cancellation was a slight possibility.
Emotional impulses and time
笑わずにはいられません。 describes an emotional reaction: being unable to suppress laughter. Unlike canceling an event because of circumstances with ざるを得ない, the focus is an internal impulse.
来月から予定を変えざるを得ません。 says that the schedule must change starting next month. Strong inevitability does not mean that the future change has already been implemented.
Quick check: What happens to 延期する in 延期せざるを得ません。?
Show the answer and why
する becomes せざるを得ない. It means having no choice but to postpone; do not use しざる.