How did weak verbs form past-tense verbs in Germanic languages?

In Germanic languages, weak verbs form their past tense through the addition of a suffix containing a dental or alveolar consonant, a feature known as the dental preterit.1

While strong verbs in Germanic languages rely on ablaut—a system of vowel gradation inherited from Proto-Indo-European—to mark tense, weak verbs represent a distinct innovation. This process involves attaching a dental suffix to the root of the verb to indicate the past tense and the past participle. For example, in Old English, the verb fyllan (to fill) forms its past tense as fylde and its past participle as gefylled.2