Tones and tone sandhi
The four tones, plus the two sound-change rules you hear every day.
Mandarin has four tones and a neutral one. Same syllable, different tone, different word.
mā, mother (high and level)
má, hemp (rising)
mǎ, horse (dipping)
mà, scold (falling)
Third tone before third tone
When two third tones meet, the first becomes second tone.
nǐ hǎo is spoken ní hǎo
不 and 一 change before a falling tone
bú duì
Pinyin on this site keeps the dictionary tone; the colours show the written tone.