Strong vs weak
Confused of Edinburgh writes:
Dear Mr Free Press
Not so much a rant but a question related to my previous one.
Why is the past tense of run RAN?
I walked down the street
I jogged down the street
I runned down the street doesn't quite work.
This is because to run is a strong verb. As a general rule, strong verbs do not take -ed to form the past tense but instead change a vowel. Other examples are the verbs to sing, to drink, and to write. Naturally, the opposite of a strong verb is a weak verb. They generally take -ed to form the past tense.
There are several classes of strong verb, characterised by the pattern of vowel changes from one tense to another. This is why drink goes to drank but think doesn't go to thank.
Over time, some verbs which used to be strong have become weak, and vice versa. The overall trend, however, is one of weakening - between two thirds and three quarters of the verbs that were strong in English a thousand years ago are now weak.
Wikipedia has a lengthy but frankly rather tedious article on the subject. I don't suggest reading it beyond the "general developments" section.
Also, I don't recommend running down the street. It's undignified. Try sauntering instead.