Cannot but / must (不得不 / 不能不 / 不可不)

Structure 不得不 / 不能不 / 不可不 + verb

All three are double negatives of necessity: they deny that something can be left undone, thereby strongly asserting that it must be done. 不得不 (bù dé bù, HSK 4) is the most common and stresses obligation under external pressure: "have no choice but to". Alongside it, HSK 6 adds two more formal variants. 不能不 (bù néng bù) = "cannot not…, one cannot help but…"; it leans toward logical or moral inevitability: 面对铁证，他不能不承认 = "faced with hard evidence, he could not but admit it". 不可不 (bù kě bù) = "must not fail to…, it is essential to…"; the most formal, typical of warnings and advice: 此中利害，不可不察 = "one must not fail to weigh what is at stake here". Register scale: 不得不 (everyday) < 不能不 (formal) < 不可不 (literary, almost sententious).

Examples

面miàn对duì铁tiě证zhèng，他tā不bù能néng不bù承chéng认rèn自zì己jǐ说shuō了le谎huǎng。
Faced with hard evidence, he could not but admit that he had lied.
此cǐ中zhōng利lì害hài关guān系xì，不bù可kě不bù察chá。
The stakes involved here must not go unexamined.
历lì史shǐ的de教jiào训xùn，我wǒ们men不bù能néng不bù深shēn思sī。
We cannot but reflect deeply on the lessons of history.
防fáng微wēi杜dù渐jiàn，为wéi政zhèng者zhě不bù可kě不bù慎shèn。
Nip trouble in the bud: those who govern must not fail to be prudent.

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