The case of "不" is an exception. Mostly when two characters of 4th tone come together, the tone stays unchanged, for example 部队,半路,半步, etc.
There are lots of exceptions. For example, generally when a character of the 2nd tone follows a 4th tone character in a word , for example 布局, the tone of the second one stays unchanged. But in the case of 事情,the tone of the second one is weakened, being pronounced like a weak tone. Similar cases are 妈妈,爸爸...
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Thanks. I think the rules are very simple. For example, if the 2nd word has 4th tune, BU should be changed into the 2nd tune. But there are so many words, it is hard to list everyone.