Regular Verbs

I need some explanation about the tenses of the verbs. I don't
understand quite well how to form the past of a regular verb
correctly. I found a page on the web with a table of forms of
conjugation for regular strong verbs but I need to know how I can
figure out to which conjugation belong a verb. I also wonder why
verbs like hata and kalla conserve the [a]of the infinitive instead
of following the umlaut.