The vowel exchange "a" to "o"

[Petrasti]

I have looked at some short examples in Arabic and Hebrew (without reference to these languages). However, it is noticeable that the inflected words change the word stems at the end of the word as well as the swapped vowel.  Or have I misunderstood this? I can't see any connection with what happened in the manuscript at the moment. Perhaps you could send me a short example of what you mean exactly? I would really like to understand this.