http://ichinesematters.blogspot.com/2016/10/tone-changes.html
1. 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)
2. 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
3. bù 不
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