Comment by grishka

That а/о thing is taught extensively in schools. There are formal rules for everything, but of course I don't remember any. What I do remember is these are called "безударные гласные" (unstressed vowels), and "проверочные слова" (test words?) are used to figure out whether it's о or а. The idea is iirc that you find a word with the same root in a different form where that syllable is stressed. Except sometimes there isn't one ¯\_(ツ)_/¯