i-mutation/i-umlaut/ front mutation
"umlaut" ,the German term means Sound Alteration. This was the most important sound change that took place in the Old English(around 7th century). This process is termed i-mutation because the change was caused by the influence of the vowel /i/. The vowels in the accented syllables were modified through the influence of /i/ in the following syllable and the /i/ subsequently disappeared.It was the back vowels or diphthongs in which one of the elements was a back vowel which were mostly changed to the front vowels through this process.
Major results of the i-mutation can be seen in the cases of the mutated plurals(eg. mus- musiz-mysiz-mys-mice), mutated abstract nouns from adjectives(lang to length),verbs from cognate nouns by mutation(food to feed, gold to gild), verbs from adjectives by mutation(eg. fullian to fill) and mutated degrees of comparison(elder ,eldest ; the eaely OE suffixe for the comparative and superlative were -ira and -ist respectively which later became -er and -est).
Though a slow process it is believed to have been well advanced by 600 AD and completed by 700 AD.
Ablaut / Gradation
The vowel changes in related forms of words especially verbs were termed "ublaut" by Javob Grimm. The term 'ablaut' literally means 'off-sounds'. It indicates a certain offing or distancing of the vowel sounds. Vowel Gradation took place in verbs according to the occurence of vowel sounds in both the stressed and unstressed syllables. All the strong verbs or irregular verbs are the examples of the vowel gradation- they are formed by changing a vowel in the base form. Eg. sing- sang- sung, ring-rang-rung. The three original grades of Ablaut were the 'e' grade, 'o' grade and'zero' grade. It has also given rise to forms like bind-band, bond.