io (eo) = Germanic eu ([§ 16]), and the io (eo) in the preterites of the old reduplicated verbs whose presents have ou, ō, uo ([§ 87]).
io (eo) = MHG. ie, as OHG. liob, dear, biotan, to offer = MHG. liep, bieten; OHG. liof, I ran, stioȥ, I pushed, riof, I called = MHG. lief, stieȥ, rief.
iu = MHG. ǖ written iu, as OHG. liuti, people, kiusit, he chooses = MHG. liute, kiuset.
ou (older au) = (1) MHG. ou, as OHG. ouga, eye, boug, I bent, loufan, to run = MHG. ouge, bouc, loufen.
ou (older au) = (2) MHG. öu (eu), as OHG. loubir, leaves, loufil, runner = MHG. löuber, löufel.
uo = (1) MHG. uo, as OHG. bruoder, brother, muoter, mother, stuont, I stood = MHG. bruoder, muoter, stuont.
uo = (2) MHG. üe, as OHG. gruoni, green, fuoȥi, feet, = MHG. grüene, vüeȥe.
Ablaut (Vowel Gradation).
[§ 12.] By ablaut is meant the gradation of vowels both in stem and suffix, which was chiefly caused by the primitive Indo-Germanic system of accentuation. See the Author’s Historical German Grammar, § 23.
The vowels vary within certain series of related vowels, called ablaut-series. In MHG. there are six such series, which appear most clearly in the stem-forms of strong verbs. Four stem-forms are to be distinguished in a MHG. strong verb which has vowel gradation as the characteristic mark of its different stems:—(1) the present stem, to which belong all the forms of the present, (2) the stem of the first or third person of the preterite singular, (3) the stem of the preterite plural, to which belong the second person of the preterite singular and the whole of the preterite subjunctive, (4) the stem of the past participle.