The tone of a word sometimes changes when used in a sentence depending on the tone of the word that preceds or comes after it.
Rule 1: If a 3rd tone is followed by a 3rd tone, the first 3rd tone becomes second tone.
Example: The word for “hello” in Mandarin is: nǐhǎo which literally means “You good.” When these two words are pronounced seperately, they are both pronounced using the third tone. When they are used together to form the word “hello” ni takes the second tone and hao remains in third tone.
Note that the word nǐhǎo is still written with two third tones, but it is said with the the nǐ in second tone.
Rule 2: If a 3rd tone is followed by any tone other than the 3rd tone (covered by rule 1) the 3rd tone changes to a “half 3rd tone.” A “half 3rd tone” is one that falls, but does not rise.
Example: ni mamma
Rule 3: The word “bu” is 4th tone, but when it is followed by another 4th tone, it becomes 2nd tone.
Rule 4: Rules concerning the word “yi.”
The word “yi” is 1st tone when used as part of a number (yi, er, san, … shiyi).
The word “yi” is 4th tone when preceeding 1st, 2nd, or 3rd tones. (yi ge ren)
The word “yi” is 2nd tone when proceeding a 4th tone