37. There are three ways of changing the singular form to the plural:—
(1) By adding -en.
(2) By changing the root vowel.
(3) By adding -s (or -es).
The first two methods prevailed, together with the third, in Old English, but in modern English -s or -es has come to be the "standard" ending; that is, whenever we adopt a new word, we make its plural by adding -s or -es.
I. Plurals formed by the Suffix -en.
The -en inflection.
38. This inflection remains only in the word oxen, though it was quite common in Old and Middle English; for instance, eyen (eyes), treen (trees), shoon (shoes), which last is still used in Lowland Scotch. Hosen is found in the King James version of the Bible, and housen is still common in the provincial speech in England.
39. But other words were inflected afterwards, in imitation of the old words in -en by making a double plural.