Grammar Lesson 11:

Tones of the character 一

The tone of 一 (yi) (meaning "one") depends on the context.

First tone is used if indicating a sequence, e.g. numbers, days of the week:

一,二,三 (yi1 , er4, san1) = one, two, three
星期一 (xing1 qi2 yi1) = Monday
第一 (di4 yi1) = first

Second tone is used if the following character uses the 4th tone:

一对/一對 (yi2 dui4) = one couple
一定 (yi2 ding4) = absolutely

Fourth tone is used if the following character does not use the 4th tone:

一起 (yi4 qi3) = together
一双 (yi4 shuang1) = one pair
一时 (yi4 shi2) = temporary