о 707 смотри здесь написано что в языке викингов был немецкий умлаут Old Norse has three categories of verbs (strong, weak, & present-preterite) and two categories of nouns (strong, weak). Conjugation and declension are carried out by a mix of inflection and two nonconcatenative morphological processes: umlaut, a backness-based alteration to the root vowel; and ablaut, a replacement of the root vowel, in verbs.