Tone sandhi is a phonological change occurring in tonal languages, in which the tones assigned to individual words or morphemes change based on the pronunciation of adjacent words or morphemes.
It is a very common phenomenon in Mandarin Chinese. Here we list some specific rules that cause tones to change in certain situations.
Tone sandhi of "不"
"不" is a fourth tone syllable by itself. But it becomes a second tone when followed by a fourth tone. For example:
Tone sandhi of "一"
"一" is normally first tone, but this changes in two situations. It is second tone when followed by a fourth tone and when followed by a first, second or third tone, it is fourth tone.
一步 yí bù
A third tone, when followed by a first, second or fourth tone, or most neutral tone syllables, usually becomes a half third tone, that is, a tone that only falls but does not rise. The tone mark is unchanged.
你哥哥nǐ gē ge
你忙吗nǐ máng ma
Image from allsetlearning.