Why do we say 'she jumped', 'he walked', 'they blinked', 'Homer nodded' but we don't say 'she thinked', 'he runned', 'they singed', and 'Homer speaked'? Or, to pose the question another way, why don't we say 'she jeamp', 'he wolk', 'they blank' or 'Homer nad'?
The answer is that English has two entirely separate systems of verbs: the 'weak' verbs, which form their past tenses by adding a dental suffix1 (usually the '-ed', although sometimes it is a '-t', or it is dropped altogether if the verb stem already ends with a dental sound); and the 'strong' verbs, which form their past tenses by a modification of the vowel sound in the root syllable of the verb.
The two types of verbs described here, strong and weak, do not comprise all verbs in the language. There is a different way of dividing the verbs into regular verbs, which comprises most of the weak verbs, and the irregular verbs, which describes the strong verbs, a number of weak verbs with irregular conjugations, and anomalous, or defective verbs. This entry is concerned only with the strong verbs and the weak verbs.
A Little History
In Anglo-Saxon times there were seven 'classes' of strong verbs, which were distinguished by the series of vowels2 that were used in the various forms of the verb. The modern English past tense of 'to sing' is 'sang', but the past tense of 'to think' is not 'thank'. This seeming difference occurs because the verbs belong to different classes of strong verbs.
In the past there were far more strong verbs in the language than there are today. Some verbs have gradually just been turned into weak verbs by the abandonment of the old system of root vowel modification and the substitution of the -ed ending. New verbs that have been coined or been borrowed from other languages have usually simply been given the weak form. An example of a strong verb that has been converted into a weak verb is 'to glide', whose past form used to be 'glad'. Rarely, a borrowed verb comes to have a strong form. One such example is the verb 'to catch' which was borrowed from French, and today has what appears to be a strong form.
Interestingly, there are a few verbs that have fluctuated between forms, for example 'to dive'. In the Anglo-Saxon period, this verb had both weak and strong forms. In much of the Modern English period, it was a weak verb. In some areas it has recently re-acquired the past tense form 'dove'. The form dove was considered only a dialectic, non-standard form just a century ago; today children3 are in many regions regularly 'corrected' if they say 'dived'.
What May Come
Throughout history English speakers have gradually been dropping the use of the strong form of verbs. Estimates vary, but there is general agreement that in the Old English period there were over 300 strong verbs. Today, there are less than one hundred, including borrowed words that have taken on strong forms. Although there are occasional complications, as in the case of to dive, it is likely that the language will continue to move toward the weak form. Someday we may all say 'I thinked about it but she had already singed it, so I taked the day off and went to the lake and swimmed.'