Grimm's law and verner's law

æ > ea before combinations of "r+consonant", "l+cons.", "h+cons.", and also before h final:
ærm > earm, æld > eald, æhta > eahta, sæh > seah
e > eo before "r+cons.", "lc, lh, h +cons.", and before h final:
herte > heorte, melcan > meolcan, selh > seolh, feh > feoh
2. Palatalization
This is the process which went under the influence of g, c, sc before vowels in the beginning of the word:
e > ie (gefan > giefan)
æ > ea (cæster > ceaster)
 æ' > éa (gæ'fon > géafon)
a > ea (scacan > sceacan)
o > eo (scort > sceort)
Interesting to know that this palatalization (or softening) is thought by some linguists to influence not vowels but consonants themselves. This means that in some particular position sounds g, c, sc became respectively [g'], [k'], [sk'], and this was marked by a soft vowel after them. So opinions vary on this problem.³
3.i-mutation This interesting feature changed many of Old English words on a very early stage of the language's history. It is caused by i (ot j) in the next syllable, it affects all vowels, except i and e. Vowels move from their back position to the new front one:
a > e (framian > fremman)
æ > e (tælian > tellan)
á > æ' (lárian>læ'ran)
o > e (ofstian > efstan)
ó > é (dómian > déman)
u > y (fullian > fyllan)
ú > ý (cúþian > cýþan)
ea > ie (earmiþu > iermþu)
éa > íe (geléafian > gelíefan)
eo > ie (afeorrian > afierran, afyrran)
éo > íe (getréowi > getríewe, getríve)
4. Back Mutation
Appears before liquids and labial consonants (i.e. r, l; p, b, f, m):