四声和变调
The four tones and sandhi

从第一课起你就在说“你好”。写的是 nǐ hǎo，两个上声；可是你嘴里出来的第一个字是往上走的，听起来像 ní。你还会说“不是”“不吃”“一个”“一起”，四个词里的“不”和“一”读得不一样。没有人教过你这几步，你却一次也没说错。这一课只是把它们写下来。

You have said 你好 since your first lesson. It is written nǐ hǎo, two third tones — yet the first syllable comes out of your mouth rising, close to ní. You also say 不是, 不吃, 一个 and 一起, and the 不 and 一 in them are not read alike. Nobody taught you those adjustments, and you have never once got them wrong. This lesson simply writes them down.

四个调都有老名字和新名字。阴平、阳平、上声、去声是从古代的四声来的；第一声、第二声、第三声、第四声是现在课本上的说法。两套名字说的是同一件事，你在字典和语音书里都会碰到。

Each tone has an old name and a new one. 阴平, 阳平, 上声, 去声 come down from the classical four tones; 第一声 to 第四声 is the modern textbook way. They label the same four things, and you will meet both in dictionaries and phonetics books.

上声在别的调前面还有一个小变化：它只降不升，听起来又低又短，这叫半三声。“老师”“美国”“可是”都是这样。半三声也不写进拼音——拼音记的是词本来的调，变调是嘴上的事。

Before any other tone, a third tone dips but does not rise again: low and short, the so-called half-third tone, as in 老师, 美国, 可是. That is not written either. Pinyin records the underlying tone of the word; sandhi happens in the mouth.

“不”还有一个轻读的地方：夹在中间的时候，比如“好不好”“看不见”“听不懂”，它读得又短又轻，拼音写 bu，不写调号。这两种情况你在第四级说“你去不去？”的时候就已经用上了。

不 also goes light when it sits in the middle: 好不好, 看不见, 听不懂 are hǎo bu hǎo, kàn bu jiàn, tīng bu dǒng, written with a toneless bu. You were already doing this back in level 4 when you asked 你去不去?

“一”夹在重叠的动词中间也读轻声：看一看 kàn yi kàn、试一试 shì yi shì。记住一条总的道理：变调是说话的规律，写不写进拼音由习惯定——“不”和“一”写，三声变调不写。

Between a reduplicated verb, 一 goes neutral too: kàn yi kàn, shì yi shì. Hold on to the general point: sandhi is a fact about speech, and whether it reaches the spelling is a matter of convention — 不 and 一 are written as changed, the third-tone rule is not.