Grammar Info
N2 Lesson 1: 3/21

ざるを得(え)ない
can't help doing, have no choice but

する→ざるを得ない, くる→ざるを得ない

Structure
Casual
Polite
Verb[ない+ ざる得ない
Verb[ない+ ざる得ません
Details
Register Formal
使用域 硬い
Information
ざる is the 連体形 (attributive form) of the old-fashioned 助動詞(じょどうし) (auxiliary verb) ざり. ざり carries the same meaning as the modern ない, in negating what comes before it.
When joined with a combination of the る-Verb 得(え)る ‘to gain’, or ‘to acquire’, and the 助動詞(じょどうし) (auxiliary verb) ない, it becomes a double-negative structure meaning that one ‘cannot help doing (A)’, or ‘there is no choice but to (A)’. More literally, it means that one ‘can’t not (A)’.
This structure will conjugate with other verbs in exactly the same way as ない.
  • 山(やま)の頂上(ちょうじょう)まで登(のぼ)りたかったが、天候(てんこう)が悪(わる)くなって来(き)たので諦(あきら)めざるを得(え)ない
    I wanted to hike to the peak of the mountain, but the weather was getting worse so we had no choice but to give up.
  • 先輩(せんぱい)に指示(しじ)されたので従(したが)わざるを得(え)ない。
    Because my senpai told me to do it, I had no choice but to obey.
Caution - As ざり is an old-fashioned structure, it conjugates a little bit differently with する than the regular ない.
  • フランスの大学(だいがく)へ行(い)くため、フランス語(ご)を勉強(べんきょう)しざるを得(え)ない。 (Unnatural Japanese)
    In order to go to a college in France, I have no choice but to study French.
  • フランスの大学(だいがく)へ行(い)くため、フランス語(ご)を勉強(べんきょう)せざるを得(え)ない。 (Natural Japanese)
    In order to go to a college in France, I have no choice but to study French.
Notice here that instead of しない changing to しざる, せざる will be used instead.
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田中(たなか)さんとても忙(いそが)しかったのでゴルフの誘(さそ)いを断(ことわ)らざるを得(え)なかった
Mr. Tanaka was so busy he had no choice but to turn down an invitation to play golf.
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今(いま)まで使(つか)った顕微鏡(けんびきょう)が壊(こわ)れたから、新(あたら)しいのを買(か)わざるを得(え)ない
The microscope I have been using until now broke, so I have no choice but to buy a new one.
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工場(こうじょう)の排気(はいき)を低(ひく)くざるを得(え)ない
We have no choice but to lower exhaust in the factory.
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Books
Tae Kim's Japanese Grammar Guide
Page 324
Dictionary of Intermediate Japanese Grammar
Page 606