A back vowel becomes a front vowel when the following syllable contains the vowel /i/ (or similar sounds) Thus the newly fronted vowel of the plural form split off from the vowel of the singular, which remained a back vowel. True, the specific vowels that strong verbs use today are different to the vowels and ablaut grades of the ancient ancestor, but the system is still there and the ways in which the vowels now differ are regular and predictable.