Origin and development
Strong verbs have their origin in the ancestral Proto-Indo-European (PIE) language. In PIE, vowel alternations called ablaut were frequent and occurred in many types of word, not only in verbs. The vowel that appeared in any given syllable is called its "grade". In many words, the basic vowel was *e (e-grade), but, depending on what syllable of a word the stress fell on in PIE, this could change to *o (o-grade), or disappear altogether (zero grade). Both e and o could also be lengthened to ē and ō (lengthened grade). Thus ablaut turned short e into the following sounds:
As the Germanic languages developed from PIE, they dramatically altered the Indo-European verbal system. PIE verbs had no tense, but could occur in three distinct aspects: the aorist, present and perfect aspect. The aorist originally denoted events without any attention to the specifics or ongoing nature of the event ("ate", perfective aspect). The present implied some attention to such details and was thus used for ongoing actions ("is eating", imperfective aspect). The perfect was a stative verb, and referred not to the event itself, but to the state that resulted from the event ("has eaten" or "is/has been eaten"). In Germanic, the aorist eventually disappeared and merged with the present, while the perfect took on a past tense meaning and became a general past tense. The strong Germanic present thus descends from the PIE present, while the past descends from the PIE perfect. The inflexions of PIE verbs also changed considerably.
In the course of these changes, the different root-vowels caused by PIE ablaut became markers of tense. Thus in Germanic, *bʰer- became *beraną in the infinitive (e-grade); *bar in the past singular (o-grade); *bērun in the past plural (ē-grade); and *buranaz in the past participle (zero-grade).
In Proto-Germanic, the system of strong verbs was largely regular. As sound changes took place in the development of Germanic from PIE, the vowels of strong verbs became more varied, but usually in predictable ways, so in most cases all of the principal parts of a strong verb of a given class could be reliably predicted from the infinitive. Thus we can reconstruct Common Germanic as having seven coherent classes of strong verbs. This system continued largely intact in the first attested Germanic languages, notably Gothic, Old English, Old High German and Old Norse.
Germanic strong verbs, mostly deriving directly from PIE, are slowly being supplanted by or transformed into weak verbs.
As well as developing the strong verb system, Germanic also went on to develop two other classes of verbs: the weak verbs and a third, much smaller, class known as the preterite-present verbs, which are continued in the English auxiliary verbs, e.g. can/could, shall/should, may/might, must. Weak verbs originally derived from other types of word in PIE and originally occurred only in the present aspect. They did not have a perfect aspect, meaning that they came to lack a past tense in Germanic once the perfect had become the past. Not having a past tense at all, they obviously also had no vowel alternations between present and past. To compensate for this, a new type of past tense was eventually created for these verbs by adding a -d- or -t- suffix to the stem. This is why only strong verbs have vowel alternations: their past tense forms descend from the original PIE perfect aspect, while the past tense forms of weak verbs were created later.
The development of weak verbs in Germanic meant that the strong verb system ceased to be productive: no new strong verbs developed. Practically all new verbs were weak, and few new strong verbs were created. Over time, strong verbs tended to become weak in some languages, so that the total number of strong verbs in the languages was constantly decreasing.
The coherence of the strong verb system is still present in modern German, Dutch, Icelandic and Faroese. For example, in German and Dutch, strong verbs are consistently marked with a past participle in -en, while weak verbs have a past participle in -t in German and -t or -d in Dutch. In English, however, the original regular strong conjugations have largely disintegrated, with the result that in modern English grammar, a distinction between strong and weak verbs is less useful than a distinction between "regular" and "irregular" verbs. Thus the verb to help, which used to be conjugated help-holp-holpen, is now help-helped-helped. The reverse phenomenon, whereby a weak verb becomes strong by analogy, is rare (one example in American English, considered informal by some authorities, is sneak, snuck, snuck. Another is the humorous past tense of "sneeze" which is "snoze").
Some verbs, which might be termed "semi-strong", have formed a weak preterite but retained the strong participle, or rarely vice versa. This type of verb is most common in Dutch:
- lachen lachte (formerly loech) gelachen ("to laugh")
- vragen vroeg (formerly vraagde) gevraagd ("to ask")
An instance of this phenomenon in English is swell, swelled, swollen (though swelled is also found for the past participle).