Hi,
Don't get into the habit of thinking of English as a kind of mathematics. It's not.
But I'm not saying that the so-called 'rules' are not important or useful, particularly as a way of beginning to learn English.
I learned to dance the Argentine tango. First, I learned the rules. Then I learned to break the rules. 

Did you look at the link I gave you about the background of irregular verbs?
Clive