The Tone Sandhi of “一”

一 as “yī”

When “一” is by itself, it’s pronounced as “yī”.

一

When “一” is at the end of a word, then it’s also pronounced as “yī”.

…… 一

e.g.

一 /yī/

三十一 /sān shí yī/

唯一 /wéi yī/

万一 /wàn yī/

一 as “yì”

When “一” is followed by a 1^st tone, 2^nd tone or 3^rd tone syllable, it’s pronounced as “yì”.

一 + 1^st tone

一 + 2^nd tone

一 + 3^rd tone

e.g.

一天 /yì tiān/

一台 /yì tái/

一种 /yì zhǒng/

一 as “yí”

When “一” is followed by a 4^th tone syllable, it’s pronounced as “yí”.

一 + 4^th tone

e.g.

一样 /yí yàng/

一对 /yí duì/

一个 /yí gè/