Starting teaching

Last night I took my first lesson. I am using the Cambridge University Press series Interchange which is really good (apart from the fact that it is American English - Janet my American co-worker would tell me off for bashing Americans again, but hey its fun!)

I have two students in my class, Richard and Pavla. They did say at the beginning of the lesson that they couldn't understand me, but I think by the end they started to understand my accent...

We looked at past tense. Now can anyone explain a logical reason why the past tense of see is saw, go is went, have is had and drive is drove?

I'm sure someone like Emma Lidbury knows the morphology of the English language... Please explain!