We all know that the plural of foot is not *foots but feet. This is, by the large majority of English teachers and speakers alike, dismissed as an irregular feature shared by a bunch of other equally irregular nouns, but the formation of the plural of foot actually only differs from the one of wolf, for example, because it originates in another basic type of noun with another internal structure and therefore also another type of regularity. The only problem is that English has, over the course of the centuries, changed so much and left so many original features behind on the road to simplification, that this other type of regularity only becomes apparent when one compares English with older related languages, such as Greek and Sanskrit.
What does basic type and internal structure mean? In older Indo-European languages all nouns are divided into two basic types according to whether or not they contain a so called theme vowel. This vowel is interposed between the root and the case endings and has no particular significance for the meaning of a word, i.e. it has no semantic value, it is simply a part of the mechanics of how language produces structurally different words, i.e. it has a purely morphological value. The morphological value of it is that it acts as a shield protecting the root from the case endings, which means that any noun containing such a theme vowel will usually not exhibit variation of the root in the different cases and numbers. The word wolf is an example for such a word. The present English form of the word has been simplified so much over time (for which c.f. the article entitled The erosion of cases), that one can no longer see that it is essentially different from a word like foot. To see this difference we have to turn to Greek and Sanskrit. In Greek wolf is λύκος (lykos), which consists of the root lyk-, the theme vowel –o– and the nominative marker –s. In Sanskrit we have an exactly corresponding structure: वृक्षः (vṛkṣaḥ, which stands for vṛkṣas) consists of vṛkṣ-, the theme vowel –a– and the ending –s. In English the theme vowel and nominative marker have been lost, but the origin of the noun can still be seen, when in the plural the typical Indo-European nominative plural marker –es appears in the expected shortened and simplified form –s.
The older of the two types are the nouns which do not include a theme vowel. They are structurally simpler, but often exhibit a more complex set of forms for the different cases. Here the stem is open to being affected by the immediate proximity of the case endings, should a language develop a sensitivity to certain sounds contained in them. The problems caused by the proximity of the case ending are probably at the origin of the development of the theme vowel. In Greek the word for foot is ποῦς (pūs) in the nominative singular, which consists of the root pod– to which has been directly added the nominative marker –s. The form pūs is caused by the fact that in Greek the –d of the root and the marker –s cannot stand directly next to each other: the –d is swallowed by the –s and the vowel is lengthened in compensation for this loss. The nominative plural is πόδες (podes), containing the typical nominative plural marker –es. During the development of the ancestor of all Germanic languages this ending changed into *-iz. This ending was already lost in Old English (for the reasons for this loss c.f. the article The erosion of cases), but the -i contained in the ending had enough time to cause a typically Germanic sound change called i-mutation before disappearing. I-mutation or umlaut, to use German term borrowed by English, consists in the sound i, which is a high vowel, i.e. is pronounced high up in the root of the mouth, pulling up a low vowel from the cellar of the mouth, so to speak, into closer proximity with its own articulation spot, which in the case of the vowel o results in e. And thus foot becomes feet in the plural. This is not only the case in English, but in all Germanic languages, e.g. in German the plural of Fuss is Füsse and in Icelandic fót becomes fætur in the plural, all of which exhibit the same signs of i-mutation, which was made possible by the absence of a protecting theme vowel and the resulting proximity of the vowel i in the original case ending of ancient Germanic.
teacher for ancient languages