Grimm's law and verner's law

A	A	Short back vowel; Mainly in open syllables, when the following one contains a back vowel; English cup
macian (to make), habban (to have)
Á	Ai	Long back vowel; In any kind of syllables; English star	stán (a stone), hátan (to call)
æ	a	Short back vowel; Met mainly in closed syllables, or in open ones, if the next syllable contains a front vowel; English bad	dæg (a day), wæter (water)
æ'	é, á	Long back vowel; as Gothic é found only in some verbal forms, as Gothic á is the result of the so-called i-mutation; German za"hlen	stæ'lon (stolen), hæ'lan (to cure)
e	i, ai, a	Short front vowel; as Gothic i, ai noticed only in some infinitives, otherwise is result of the mutation of i; English bed	sengean (to sing)
é	ó	Long front vowel; resulted from the i-mutation of ó; German Meer	déman (to judge)
I	i, ie	Short front vowel; can be either stable or unstable, the unstable sound can interchange with ie and y; English still	bindan (to bind), niht - nyht (a night)
Í	Ie	Long front vowel; also stable and unstable (mutating to ý); English steal	wrítan (to write), hí - hý (they)
o	u, au	Short back vowel; English cost	coren (chosen)
Ó	O	Long back vowel; English store	scóc (divided)
u	u, au	Short back vowel; used only when the next syllable contains another back vowel; English book	curon (they chose)
ú	ú	Long back vowel; English stool	lúcan (to look)
y	u	Short front vowel; i-mutation of u; German fu"nf	gylden (golden)
Ý	Ú	Long front vowel; i-mutation of ú, German glu"hen	mýs (mice)
a.	O	A special short sound met only before nasals in closed syllables	monn (a man)

 
Here they are. Some linguists doubt about the last vowel if it ever existed at all - Old English texts never reflect it in writing. But there is a vowel interchange in some kind of words - in closed syllables before nasals - where some texts have a (mann), but some prefer o (monn). Sometimes even the same document shows the two variants. This makes us think there was a vocal sound in this position which was developed already in the Old English time, and did not exist in Gothic or in Common Germanic.
The Gothic language used to ²have much more diphthongs than Old English. Usually this is the general trend in Indo-European languages - diphthongs turn into simple vowels, and the more developed the language, the less diphthongs it has. The Old English tongue had two original diphthongs, both of which were composed of long vowel components:
éá - equal to Gothic au, found both in nominal and verbal parts of speech: béám, céás (he chose)
éó - equal to Gothic iu: céósan (to choose); in some dialects and varieties of the language it was written like íó, but we are sure this was the same sound in fact.
One of the main phonological and morphological instruments in Common Germanic and practically in all Germanic languages was the Ablaut, the vowel interchange in the root of nouns and verbs. This specific feature, though known in all Indo-European groups as a phonetic means, was of great importance particularly in Germanic, where it was sometimes used instead of verb endings and noun inflections. Interesting, that the same way of "infixation" of different vowels into the root is known in Semitic and other Afroasiatic languages: compare the Arabic language which has kataba (he wrote), kutiba (written), katib (writing), kitab (a writing), 'aktaba (he made smb write) as different forms of the root k-t-b, and the English language which uses sing, sang, sung, song as different forms of the root s-ng. This is the Germanic Ablaut.
The Ablaut in its classical Germanic forms is present in Gothic, Old High German and many other ancient Germanic languages. But English, though keeping this interchange, slightly changed the rules of the Ablaut. For instance, if the Germanic classical verb conjugation represent the Ablaut in i - a - zero forms, in Old English it looks like rísan - rás - rison - risen (to rise) in the I class of Strong verbs, and like béodan - béad - budon - boden in the II class. But still the Ablaut played an important role in the morphology of the verb and noun (for nouns it could be also the means of word composition: faran (to go, to travel) produced the noun fór (a trip)).
Speaking about vowel mutations which took place in Old English words through its period of existence, we do not wish to describe thoroughly all of them, just to point the most important processes:
1. Breaking