This gets a little rough, so y'all might want to fasten your seatbelts and return your seatbacks and trays to the upright position.
In Modern English we classify verbs as regular or irregular. In Old English they were classified as strong or weak. There is a rough correspondence of weak to regular and strong to irregular but this gets a little confused by verbs that have switched teams in the middle of the stream. Nuncle can probably give us some examples off the top of his head but I'll leave them be for now. This correspondence is not one-to-one. Some examples of weak verbs that we would call irregular are seek/sought, think/thought, and work/wrought. This last is one that has become regularized, the past tense usually being worked. The other changes, the vowel changes, the loss of the n in think/thought, and the change of k to gh (the gh eventually becoming completely unpronounced in Standard English), are the result of other linguistic forces. The postion shift of the r in wrought is, as fredbide has explained, metathesis. But it's the final t that lets us know that the verb is weak.
