Linguistic deveopments

Many linguistic developments can be found during the Old English period: changings in the vowel system, in consonants, in nouns, adjectives or verb. Moreover, the most important sound change which happened in this period is called “I mutation”. It is also known as “i-umlaut” (meaning alteration of sounds). With this sound change, most of the vowels and diphthongs were modified if the vowel “i” or the related consonant –the semivowel “j”- stood in the following syllable.
If in a word the 2^nd syllable contains an “i”, the vowel or diphthong before is going to approximate its pronunciation to a position closer to that “i”. The main problem is that neither “i” nor “j” survived to the time of our texts; they were lost before surviving reconrds were written down. And its original presence can be established by an examination of the corresponding words in related language.
This sound change deals with a change from back to front vowels. Back vowels are “a”, “o” and “u”. The rest of vowels are front. 

This phonological change has had profound morphological consequences, and the most relevant one was the mutation of plurals.