日本語

Damn, 1st time I've ever posted in the text boards...
Anyway, wondering if anyone could point me in the direction of a good resource on euphonic changes in Japanese (i.e. why 一本 becomes ippon and not ichihon)... I have a general understanding but was wondering if their's an established set of rules to follow. Looked around and nothing so far...

Today my sensei said that Japanese originally had it's own way to count then they took up some chinese readings in order to make their lives more difficul- er, because they were culturally impacted?