Sound Changes Relevant to Old English

I-Mutation
The i-mutation was a vowel-mutation that has been observed to affect many Old English words.

Cause and Effect
First, to understand it better, you must be aware of the different parts of the mouth in which Old English vowels were pronounced. Here is a table which shows where the vowels were pronounced (vertical: low, central, high; horizontal: front, back):

The i-mutation happened sometimes when the high front vowel (e.g. i) or its corresponding consonant (e.g. the y in yes) was after the main vowel of a word (often in suffixes); the speaker's mind would sometimes get ready to pronounce the high front vowel too soon and would affect how speaker said the main vowel, causing back vowels to shift forward to their corresponding front vowels (note that u would shift to y and keep its rounding, not i) and front vowels (if they could) would shift upwards (obviously i could not shift any further upwards, so it was unaffected), and often the i/y sound was lost. Here's a more detailed chart of the effect:

Note that all of the back vowels, whether long or short, could be affected by the i-mutation, but only the short front vowels could be affected.

Note also that a, when before a nasal consonant (m or n) would "skip a corner" and shift to e, therefore:

The two diphthongs eo and ea shifted to ie: