There are certain situations, normal tone of a Chinese character will change to another tone according to what comes before or after it.
Three main tone change rules you need to know as follows:
Rule 1: Tone changes for third tones.
Rule 2: Tone changes for yi (means one).
Rule 3: Tone changes for bù (means “no”).
Interestingly, English is that in English intonation or tone often functions at the sentence level whereas Chinese is at the word level.
Normally, the tone changes are not written in the Chinese pinyin in order not to confuse the “normal” tone of a Chinese character.