阿: when is it pronunced 'a' when is it pronunced 'e'?
Yes, there are rules - it is not irregular at all.

The two different pronunciations of 阿 actually reflect different usages. The original Old Chinese pronunciation of ē is for using the character as a verb or noun, which is what the character 阿 originally meant. In contrast, the later pronunciation of ā emerged as a particle with completely different meanings.

Unfortunately most people have became unfamiliar with the older usage, so that the ē form is usually only seen in set phrases (成語).



阿 ē


verb, meaning:


to pander to
to (unjustly) side with (someone in a dispute)

noun (archaic), meaning:


a hill
a (geographical) twist/turn
a building

A surname




阿 ā


prefix for names or familial titles, originally meaningless but can carry connotations of:


familiarity
endearment

particle (archaic, superseded by 啊) indicating:


surprise
doubt





As the above shows, the two different pronunciations do not overlap in their usage.