Mandarin Chinese is a tonal language, which means that the pitch contour of a syllable changes the meaning of a word. There are four main tones plus a neutral (light) tone. The first tone is high and level, the second tone rises, the third tone falls and then rises, and the fourth tone falls sharply. The neutral tone is short and unstressed, and it appears often on the second syllable of reduplicated words and on grammatical particles. For example, the greeting 你好 (nǐ hǎo) is written with two third tones, but in natural speech the first third tone often changes to a second tone, so it sounds like ní háo. Similarly, 谢谢 (xièxie) is two fourth tones, but the first syllable becomes lighter in connected speech.
Counting in Mandarin starts with the simple characters 一 (yī) through 十 (shí), covering the numbers one to ten. The character 一 has flexible pronunciation: it is yī on its own, yí before any non-fourth tone, and yì before a fourth tone, as in 一二 yī èr and 一样 yí yàng. The number two exists in two forms. The formal or written form is 二 (èr), used in dates, ordinals, and standalone counting, as in 第二 (second). The form 两 (liǎng) is used before measure words and in casual speech, as in 两个人 (liǎng ge rén, two people). Numbers eleven through nineteen combine 十 with the digit, such as 十一 (shí yī) and 十九 (shí jiǔ). Tens like 二十 (èr shí), 三十 (sān shí), and 四十 (sì shí) follow the pattern digit plus 十. Hundreds and thousands extend the pattern: 一百 (yī bǎi) is one hundred and 一千 (yī qiān) is one thousand.