1. 一 is pronounced as the 1st tone “yī” when
- it’s by itself,
- it works as a number,
- it is put at the end of the word, phrase or sentence.
唯一 (wéi yī) the one and only
2. 一 is read as the 4th tone “yì” when it’s followed by a 1st tone, 2nd tone or 3rd tone syllable.
|一 + 1st Tone||一斤（yì jīn）|
|一 + 2nd Tone||一台（yì tái）|
|一 + 3rd Tone||一把（yì bǎ）|
3. 一 becomes the 2nd tone “yí” when it precedes a 4th tone syllable.
|一 + 4th Tone||一样 （yí yàng）|