Grimm's law and verner's law

i > io (hira > heora)
e > eo (herot > heorot)
a > ea (saru > searu)
5. Contraction of vowels due to a dropped h
After the consonant had dropped, two vowels met, and they collided into one long vowel:
ah + vowel > eah + vowel > éa, (slahan > sleahan > sléan)
eh,ih + vowel > éo (sehan > seohan > séon)
oh + vowel > ó - (fóhan > fón, hóhan > hón)
Once I saw this feature was called the "secondary laryngeal drop" in one book. Really, this reminds the common situation of the Proto-Indo-European laryngeal sound [h] which was dropped in all Indo-European languages except Anatolian (Hittite hartagga, Greek arktos, Welsh art - a bear). The drop of this sound usually caused the lengthening of neighbouring sounds. We see that this trend was preserved in Old English as well - h was not stable enough to remain between vowels. The English language was moving towards the analytism.
The consonants in the Old English language are simple to learn for a nowadays English-speaker - and we are all, aren't we? They look the following way:

Labials	p, b, f, v
Dentals	d, t, s, þ (English thin), ð (English this)
Velars	c [k], g, h
Liquids	r, l
Nasals