Rules concerning the pronunciation of the character yī 一（one):
The character yī 一 （one）is pronounced as the 1st tone when used as part of a number.
E.g. yī 一 （one), èr 二 （two), sān 三 （three)…
1) Yī 一 （one） is pronounced as the 4th tone when preceding 1st, 2nd, or 3rd tones.
E.g. Yī tiān is pronounced yì tiān 一天 (one day)
Drills:
一只鸡 yì zhī jī, a chicken
一家人 yì jiā rén, a family
一间教室 yì jiān jiàoshì, a classroom
一包烟 yì bāo yān, a pack of cigarettes
一节课 yì jié kè, a class
一瓶酒 yì píng jiǔ, a bottle of wine
一条河 yì tiáo hé, a river
一头牛 yì tóu niú, a cow
一口人 yì kǒu rén, a family member
一碗饭 yì wǎn fàn, a bowl of rice
一所学校 yì suǒ xuéxiào, a school
一种语言 yì zhǒng yǔyán, a language
2) Yī is pronounced as a 2nd tone when preceding a 4th tone.
E.g. Yīcì is pronounced yícì 一次 （one time, once）
Drills:
一位老师 yíwèi lǎoshī, a teacher
一个哥哥 yígè gēgē, an older brother
一座大楼 yízuò dàlóu, a building
一块橡皮 yíkuài xiàngpí, an eraser