In the Germanic languages, a strong verb is a verb that marks its past tense by means of changes to the stem vowel. A minority of verbs in any Germanic language are strong; the majority are weak verbs, which form the past tense by means of a dental suffix.
In modern English, strong verbs include sing (present I sing, past I sang, past participle I have sung) and drive (present I drive, past I drove, past participle I have driven), as opposed to weak verbs such as open (present I open, past I opened, past participle I have opened). Not all verbs with a change in the stem vowel are strong verbs, however: they may also be irregular weak verbs such as keep, kept, kept or bring, brought, brought. The key distinction is that the system of strong verbs has its origin in the earliest sound system of Proto-Indo-European, whereas weak verbs use a dental ending (in English usually -ed or -t) that developed later with the branching off of Proto-Germanic.
