Tone change rule
The shape-shifting 一
一 (yī) becomes yí before a falling tone, and yì before everything else.
Written
Spoken
yī + 4th → yí ｜ yī + 1st/2nd/3rd → yì ｜ counting stays yī
The number one is the biggest shape-shifter in Mandarin. Alone, or in numbers and dates, it is a flat yī. Before a falling tone it rises: yí. Before first, second or third tones it falls: yì. Three costumes, one character.
Dates and plain counting keep the flat tone: yī yuè (January), dì yī (first). The costume changes only when 一 works as 'a' or 'one of something'.
Words you already know, doing it
一个 yī gè yí gè one (of something)
一半 yī bàn yí bàn half
一天 yī tiān yì tiān one day
一起 yī qǐ yì qǐ together
一月 yī yuè yī yuè January (no change: it is a date)