Pronouncation "Tone sandhi", most important rules (tones: 1=high, 2=up, 3=low, 4=down):
If 2 consecutive characters have the tone numbers 3 and 3, they mostly get pronounced as 2 and 3
If 2 consecutive characters have the tone numbers 3 and 1, they mostly get pronounced as 2 and 1
If 2 consecutive characters have the tone numbers 4 and 4, they mostly get pronounced as 2 and 4
If 一 (yī) is part of a word or phrase and isn’t the last character of the word/phrase or if it’s used with a measure word or a digit of a number until 99, it’s the fourth tone: 一点儿 (yì diǎnr) - a little, or 一起 (yì qǐ) - together. But when it should have been changed to tone 4, and afterwards there is another tone 4, it becomes tone 2, like in 一样 (yí yàng, same/the same) or 一定 (yí dìng, definitely)
Sometimes, 不 (bù) changes from the fourth tone to the second tone, when it is followed by another fourth tone character. Common examples are: 不会 (bú huì, will not) or 不要 (bú yào, to not want or 不是 (bú shì, to not be)