Weak verbs should be contrasted with strong verbs, which form their past tenses by means of ablaut (vowel gradation: sing - sang - sung). Most verbs in the early stages of the Germanic languages were strong. However, as the ablaut system is no longer productive except in rare cases of analogy, almost all new verbs in Germanic languages are weak, and the majority of the original strong verbs have become weak by analogy.
Strong to weak transformations
As an example of the rather common process of originally strong verbs becoming weak, we may consider the development from the Old English strong verb scūfan to modern English shove:
scūfan scēaf scofen (strong class 2)
shove shoved shoved
Many hundreds of weak verbs in contemporary English go back to Old English strong verbs.
In some cases, a verb has become weak in the preterite but not in the participle and may be thought of as "semi-strong" (not a technical term). Dutch has a number of examples:
wassen waste gewassen ("to wash")
lachen lachte gelachen ("to laugh")
An example in English is:
sow sowed sown (strong class 7 with weak preterite)
Often, the old strong participle may survive as an adjective long after it has been replaced with a weak form in verbal constructions. The English adjective molten is an old strong participle of melt, which is now a purely weak verb with the participle melted. The participle gebacken of the German verb backen (to bake), is gradually being replaced by gebackt, but the adjective is always gebacken (baked).
Weak to strong transformations
The reverse process is very rare and can also be partial, producing "semi-strong" verbs:
show showed shown (originally weak verb with participle modelled on sown)
Weak verbs which develop strong forms are often unstable. A typical example is German fragen (to ask), which is historically weak and still weak in Standard German, but for a time in the 18th century, the forms fragen frug gefragen by analogy with for example tragen (to carry) were also considered acceptable in the standard. They survive today (along with a present tense frägt) in the Rhinelandic regiolect and underlying dialects. In Dutch, the new strong past vroeg of the cognate vragen is standard today, but its past participle is weak gevraagd (though some dialects do have gevrogen).