“Strong Verb: A term in the description of Germanic languages for a verb that indicates such differences as tense by modifying its vowels: English ring, rang, rung. In contrast, weak verbs add inflections: play, played, played. These terms are usually replaced in grammars of modern English by regular verb (in place of weak verb) and irregular verb (in place of strong verb). In Old English, strong verbs could have as many as four different vowels, since the first- and third-person singular in the past differed from all the other past forms: compare was and were in the Modern English past of the verb be. An example from Old English is the verb helpan, with e in the present tense, but past healp (first- and third-person singular) and hulpon, and the past participle holpen (with the -en inflection found in some Modern English irregular verbs: shaken, taken). In Modern English, this verb has become weak (help, helped), a change that his affected many other strong verbs over the centuries, such as climb, step, walk. The strong verbs that have survived into Modern English seldom retain the original distinctions, and all (except the highly irregular be, with was and were) have lost the two forms for the past. In some Modern English verbs, the vowels or the past and the past participle have become identical (sting, stung), and in others all three forms are the same (put). Some originally strong verbs have regular variants (swell, swelled, or swollen). A few originally weak verbs have become strong, such as wear, dig, fling. Differences may occur between varieties: (1) dive, dived in British English, but often dive, dove in American English; (2) sell, sold and tell, told in standard English worldwide, but sell, sellt, tell, tellt in Scots. Occasionally, for facetious purposes, people play with strong forms: I thunk very hard about it and Where were you brung up? In general, new verbs in Modern English are regular; that is, formed on the pattern of weak verbs, the pronunciation of the -ed inflection as /(e)d/ or /t/ varying systematically according to the immediately preceding sound. Verbs formed by prefixation or compounding usually take the same forms as the verbs on which they are based: offset, babysit, and (both regular and irregular) deepfreeze. Some phrasal verbs prefer a weak form (contrast The card sped up the hill and The car speeded up).”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.