Double negatives: 不得不, 非…不可, 不是不

Two negatives, one strong positive

In Chinese, two negatives in one sentence cancel out, and the result is usually stronger than the plain positive. 我不能不去 ("I can't not go") is stronger than 我要去 ("I'm going").

不得不: forced by circumstances

不得不 says the situation leaves you no choice, usually something you'd rather not do.

• 下雨了，我们不得不取消比赛。It started raining, so we had no choice but to cancel the match.

非…不可: determination or absolute necessity

非 + verb + 不可 says something must happen, either because it's necessary or because someone insists. In speech you also hear 非得 (fēiděi) …不可, or 非要 + verb ("insist on").

• 这件事我非告诉他不可。I absolutely have to tell him about this.

不是不: softening

不是不 doesn't make a strong positive: it corrects a wrong impression, and a "but" usually follows.

• 我不是不想帮你，是真的没时间。It's not that I don't want to help you; I really don't have time.

没有…不 and 不能不, 不会不

• 这儿没有人不认识王老师。Everyone here knows Teacher Wang.
• 朋友结婚，我不能不去。My friend is getting married; I can't not go.
• 他每天都来，今天不会不来的。He comes every day; he's bound to come today.

没有…不 links up with Question words as 'any' and 'every': 谁都, 什么都不, 怎么也 (谁都 = everyone) and 连…都/也: even.

Common mistakes

• 我明天不得上班。 (meaning "I have to work") → 我明天不得不上班。 不得 with a single 不 means "must not, is not allowed", as on signs: 不得吸烟 = no smoking.
• 我不得不去不可。 → 我不得不去。 or 我非去不可。 Two patterns mixed together.
• 我非去。 → 我非去不可。 In the standard pattern 非 needs 不可 at the end. (Colloquial speech does use bare 非 or 非要 for "insist on": 他非要去, 我就非去！ Learn the full pattern first.)
• Reading 我不是不喜欢 as 我不喜欢. It is softer: "It's not that I don't like it (but…)".