Mandarin Chinese
Mandarin features four sandhi tone rules.
1. When there are two 3rd tones in a row, the first one becomes 2nd tone, and the second one becomes a half-third tone. E.g. 你好 (nǐ + hǎo = ní hǎo)
2. The neutral tone is pronounced "low" when following the 1st, 2nd, and 4th tones, and pronounced "high" following the 3rd tone.
3. 不 (bù) is 4th tone except when followed by another 4th tone, when it becomes second tone. E.g. 不对 (bù + duì = bú duì)
4. 一 (Yī) is 1st tone when alone, 2nd tone when followed by a 4th tone, and 4th tone when followed by any other tone. Examples: 一个 (yī + gè = yí gè), 一次 (yī + cì = yí cì), 一半 (yī + bàn = yí bàn), 一般 (yī + bān = yì bān), 一毛 (yī + máo = yì máo), 一会儿 (yī + huìr = yí huìr).
Read more about this topic: Tone Sandhi