How 一 (yī) and 不 (bù) Change Their Tone
The tones of 一 (yī) and 不 (bù) depend on the syllable after them. Two short rules make them sound natural.
Quick answer
Before a fourth tone, 一 and 不 rise: yíyàng, bú shì. Before any other tone, 一 falls (yì tiān) and 不 stays bù.
Hear the phrase, then repeat it aloud.
When to use it
The change happens every time you speak
, not only in careful speech. 一 (yī) keeps its own tone when you count or read out digits, in ordinals, and at the end of a word: 十一 shíyī, 第一 dì-yī, 唯一 wéiyī.
Common mistake
Reading the dictionary tone aloud. 不是 said as bù shì, with two falling tones, sounds stiff; let 不 rise into bú shì
.
Practise aloud
You’ve got this!
The rule looks at the next syllable’s dictionary tone, even when that syllable is said lightly. 个 is gè in the dictionary, so 一个 is yí ge, although 个 itself sounds light; 一下 works the same way: yíxià, often said yíxia.
Inside set phrases and yes-or-no questions like 好不好, 不 is said lightly and written without a tone mark: 对不起 duìbuqǐ, 好不好 hǎo bu hǎo, 听不懂 tīng bu dǒng.
Some books print the spoken tone (yí ge, bú shì); dictionaries often show the original one (yī gè, bù shì). Whichever you see, say the spoken one.