Tone changes: two third tones, 一 and 不

你好 is written nǐ hǎo but said ní hǎo, because the first of two third tones in a row rises instead of dipping.

Dictionaries give each character the tone it has when said alone. In connected speech a few tones change depending on the syllable that follows. Linguists call this tone sandhi, and Chinese textbooks call it 变调 biàndiào. Three rules cover almost everything a beginner meets: the rule for two third tones, the rule for 一 yī (one) and the rule for 不 bù (not). The changes are not written in standard pinyin. Dictionaries print the original tone, and you are expected to apply the rule as you speak, so it pays to learn the rules early, before the written tones settle in your ear.

Two third tones in a row

When a third tone is followed by another third tone, the first one is said as a second tone. 你 nǐ and 好 hǎo are both third tones, so 你好 comes out as ní hǎo. In the same way 可以 kěyǐ (may) is said kéyǐ, 很好 hěn hǎo (very good) is hén hǎo, and 水果 shuǐguǒ (fruit) is shuíguǒ. Try saying two full dips in a row and you will feel why: the voice has to fall, rise, fall and rise again in a split second. Changing the first to a rise makes the pair easy to say. When three or more third tones come together, speakers group them into chunks and apply the rule inside each chunk, so 我很好 (I am fine) is often said wǒ hén hǎo, and at speed wó hén hǎo.

Even when no other third tone follows, a third tone inside a phrase usually loses its rise. Before a first, second or fourth tone it is said as a low, slightly falling pitch, sometimes called the half third tone. 老师 lǎoshī (teacher) starts low and jumps up to the level shī; it does not dip and climb first. Only at the end of a phrase, or when you say a word slowly and carefully, does the full 214 appear. That is why unit 1 suggested aiming low first.

一 yī, one

一 has three faces. It is the first tone yī when you count, when it is part of a number like 十一 shíyī (eleven) or 第一 dì yī (first), and at the end of a word. Before a fourth tone it becomes second tone: 一个 yí ge (one of), 一样 yíyàng (the same), 一半 yíbàn (half). Before a first, second or third tone it becomes fourth tone: 一天 yì tiān (one day), 一年 yì nián (one year), 一起 yìqǐ (together). Between a repeated verb it goes light, so 看一看 (take a look) is kàn yi kàn. A useful check: the classifier 个 is a fourth tone in the dictionary, gè, which is why 一个 is yí ge even though 个 itself usually sounds neutral there.

不 bù, not

不 is simpler. It is a fourth tone, bù, everywhere except before another fourth tone, where it becomes second tone: 不是 bú shì (is not), 不对 bú duì (not right), 不要 bú yào (do not want). Before the other tones it stays bù: 不好 bù hǎo (not good), 不忙 bù máng (not busy), 不多 bù duō (not much). Inside some set phrases and yes or no questions it goes light, as in 对不起 duìbuqǐ (sorry) and 去不去 qù bu qù (going or not). Some learner textbooks print the changed tones, such as yí ge and bú shì, to help beginners, so you will see both spellings; dictionaries keep yī and bù.