http://ichinesematters.blogspot.com/2016/10/tone-changes.html
Tone sandhi [3] [3] ⇒ [2] [3] 1) 语法yǔfǎ (grammar) 2) 雨伞yǔsǎn (umbrella) 3) 保养Bǎoyǎng (maintenance) 4) 洗脸xǐliǎn (wash face) 5) 广场guǎngchǎng (public square) 6) 老鼠lǎoshǔ (mouse) 7) 老虎lǎohǔ (tiger) 8) 勇敢yǒnggǎn (brave)
yī 一（one）
The word yī 一 （one）is 1st tone when used as part of a number.
E.g. yī 一 （one）, èr 二 （two）, sān 三 （three）…
• But yī 一 （one） is pronounced with 4th tone when preceding 1st, 2nd, or 3rd tones.
E.g. Yī tiān is pronounced yì tiān 一天 (one day)
Drills: yì zhī jī 一只鸡 a chicken
yì jiā rén 一家人 a family
yì jiān jiàoshì 一间教室 a classroom
yì bāo yān 一包烟 a package of cigarettes
yì jié kè 一节课 a lesson
yì píng jiǔ 一瓶酒 a bottle of wine
yì tiáo hé 一条河 a river
yì tóu niú 一头牛 a cow
yì kǒu rén 一口人 a person in a household
yì wǎn fàn 一碗饭 a bowl of rice
yì suǒ xuéxiào 一所学校 a school
yì zhǒng yǔyán 一种语言 a kind of language
• Yī is pronounced with 2nd tone when preceding a 4th tone.
E.g. Yīcì is pronounced yícì 一次 （one time, once）
Drills: yí wèi lǎoshī 一位老师 a teacher
yí gè gēge 一个哥哥 an older brother
yí zuò dàlóu 一座大楼 a big building
yí kuài xiàngpí 一块橡皮 an eraser
The word bù 不（no） has the 4th tone, but when it is followed by another 4th tone, it becomes 2nd tone.
E.g. bù shì 不是 （is not） is pronounced búshì
Practices: bù duō 不多 not a lot
bù'ān 不安 unease
bù zhīdào 不知道 do not know
bùnéng 不能 can not
bùxíng 不行 no/ it does not work. (colloquial)
bùchéng 不成 no/ it does not work. (dialect)
bù míngbái 不明白 do not understand clearly
bù hǎo 不好 not good
bù xǐhuān 不喜欢 do not like
bù kěyǐ 不可以 can not (to be not allowed to do something)
bù dǒng 不懂 do not understand
búshì 不是 to be not
búhuì 不会 can not (do not know how to do something)
búqù 不去 do not go
búpà 不怕
not afraid