Basic Rules of Modified Tones
Rule 1: It is difficult to pronounce third tone fully when two of them are together. So when two third tones are placed next to each other, we read the first third tone as a second tone.
Rule 2: When“bu” is alone or before the first tone, second tone and third tone, we read bù, the fourth tone.
xīng qī yī — MondayWhen “yi” is used before the fourth tone, it is pronounced as the second tone,yí.yí kuài qián — one yuanWhen yi is used before other three tones, it is pronounced as the fourth tone,yì.yì tiān — one day; yì bǎi — one hundredRule 3: “yī” means one. When “yi” is used alone or before nothing, it is pronounced as the first tone,yī.
Introduction to Chinese: Pronunciation and Tone

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