The tone that 一 is pronounced with is sometimes hard to predict. Is there a good general rule for when to use yi1 and when to use yi2/yi4?
The 不 tone change rule is fairly straightforward and easy to remember. 不 is underlyingly
bu4 but changes to
bu2 before another fourth tone, within a phrase. Maybe the 不-change rule can be blocked by a phrase boundary; I am not sure.
The 一 tone change rule (YTC) is much more confusing. 一 can be pronounced as
yi4 according to both the surrounding environment and how 一 is being used.
For explaining the YTC, it almost seems like there are two separate words: one of which is always pronounced
yi1 and the other is pronounced as
yi2 using the same rules as 不.
yi4 pronunciation appear to be in complementary distribution and never appear phrase-finally.
yi1 seems to appear in contexts where the corresponding two-related lexeme would use 二 instead of 两.
I am not sure that the distribution of 两 versus 二 is the exactly the same as yi2/yi4 versus yi1. Is there a better rule of thumb?