How did strong verbs use vowel changes to mark past tense in Germanic languages?

In Germanic languages, the "strong" verb system marked the past tense by transforming the Proto-Indo-European ablaut—a system of vowel alternations—into a specific, systematic tense marker.1

While Proto-Indo-European roots could appear in various grades (such as e-grade, o-grade, or zero-grade) during inflection and word formation, Germanic languages repurposed these inherited vowel variations to distinguish between present and past forms. For example, in the verb "bear," the Proto-Indo-European roots (bher-, bhor-, bhēr-, bhṛ-) were utilized to create distinct forms: the present beran, the past singular bǣr, the past plural bǣron, and the past participle boren.12

This mechanism allowed Germanic languages to shift from the complex Proto-Indo-European tense-aspect system to a simpler contrast between present and past tenses. Additionally, some strong verbs underwent consonant alternations—such as the shift between s and z—due to Verner’s law, which was triggered by the position of the accent in the original Proto-Indo-European language.1