There’s no satisfactory explanation for the essence of this type of mutation. This is an example of such a change.
naht>neaht, nieht, niht, nyht ‘night’
It may be due to a palatal quality of the h or due to an i which was probably a case ending (падежное окончание) in the original type of declension (склонение) to which the word belonged.
Quantitative Changes in OE Vowels
Quantitative changes in OE vowels are represented by lengthening.
Lengthening took place in certain positions in the 9th century.
1. Vowels are lengthened before the clusters ld, nd, mb:
cild>cīld ‘child’
bindan>bīndan ‘bind’
climban>clīmban ‘climb’
But if the cluster was followed by another consonant, lengthening did not take place, as in cildru ‘children’.
LENGTHENING
2. It took place if some consonants were lost.
a) The vowels were lengthened when ʒ was lost before d,n.
sæʒde>sǣde ‘said’
mæʒden>mǣden ‘maiden’
b) The vowels were lengthened when m,n were lost before f, s, Þ, h.
3. Lengthening took place in case of contraction (стяжение). If, after a consonant had dropped, two vowels met inside a word, they usually contracted into one long vowel.
