• The tone of a word sometimes changes when used in a sentence, depending on the tone of the word that precedes or comes after it.
Rule 1• If a 3rd tone is followed by a 3rd tone, the first 3rd tone becomes a 2nd tone (3+3 → 2+3).
Example: nǐ hǎo → ní hǎo (hello) hěn hǎo → hén hǎo (very good) hǎo jiǔ → háo jiǔ (good wine) shuǐ ɡuǒ → shuí ɡuǒ (fruit)• Note: The word is still written with two third tones, although the first syllable is read in second tone.
Rule 2• The word bù 不（no; not） has the 4th tone, but when it is followed by another 4th tone, it becomes 2nd tone. (4+4 → 2+4)
Example: bù cuò → bú cuò (not bad) bù qù → bú qù (not go) bù duì → bú duì (wrong) bù dàn → bú dàn (not only)• Note: To simplify, “不” is marked in the fourth tone “bù” in Chinese dictionary entries. In Chinese textbooks, on the other hand, it is marked according to the actual pronunciation.
1. The word yī 一 （one）is 1st tone when:①yī comes at the end of a word. Example: wàn yī : in case wéi yī : only②it acts as an ordinal number to indicate the number “one.” Example: yī bān : Class One yī nián jí : Grade One yī hào : the first day of month dì yī mínɡ : the first place
2.yī is pronounced with 4th tone when preceding 1st, 2nd, or 3rd tones.（yī → yì） Example: yī tiān → yì tiān (one day) yī zhí → yì zhí (always) yī bǎi → yì bǎi (hundred)
3.yī is pronounced with 2nd tone when preceding a 4th tone. （yī → yí) Example: yī cì → yí cì (one time, once) yī ɡònɡ → yí ɡònɡ (altogether) yī wàn → yí wàn (ten thousand)