Why is the preterite of run ran, yet the preterite of shun is shunned?I do not know the actual answer to this question, but there are normally two explanations for this, both working together. The change of vowel sound: run->ran is generally derived from Arabic, while the change of ending is a European (Greek, Latin) technique.

Euh, sorry to correct you here, but the changing vowel is a typical feature of Germanic language. It's the difference between strong verbs and weak verbs, one of the two retains their vowel but changes the ending, the other changes the vowel.

In the above example: "I run, "I ran" would be in German "Ich renne", "Ich rannte". Pretty similar eh? "I can, I could" -> "Ich kann, Ich konnte". In some cases the English verbs have lost their irregularities (like English lost a lot of Germanic grammar): "I help, I helped" -> "Ich helfe, Ich half". Although the regularization of verbs is a general trend in German too.

Glad I could help :-) but IANAL (linguist)