> > And while the question is being considered, how about
> >
> >goose geese
> >mouse mice
> >louse lice
>
> That one's rather older: Germanic i-umlaut before the consonant stem
> plural ending *-iz (< PIE *-es).
I'm comfortable with my understanding of umlaut, but it took a while.
MCV's remark is correct of course, but it does not answer the basic
native-speaker's question of *why* English has these bizarre fossil
plural forms.
Umlaut is not something that is easily explained to monolingual
native-speakers of English, particularly the AmE variety. Umlaut
describes a phonological process which is now 100% dead in English,
and, so I am told, is just barely alive, and only via analogy, in
German.
This question, "why 'feet' as the plural of 'foot'?", is at some point
asked by every native-speaker; most of us put it aside, never
receiving an adequate answer. Even our teachers don't understand.
If you learn Turkish, of course, you'll experience a great awakening.
Vowel Harmony! But once again, no monolingual native-speaker of AmE
will ever understand vowel harmony, except in an abstract sense. You
accept on faith the explanation that Turkish-speakers do bizarre
things with their vowels, making the first vowel in a word 'harmonize'
with that word's second vowel, i.e., the first vowel ignores it's
etymological vowel and substitutes the second vowel.
It's (approximately) the Old English plural of 'foot', being logically
formed as 'footies' (at least to our modern native-speaker
sensibilities), and then becoming 'feeties' because the first vowel
must take on the value of the second, and then shortening itself to
'feet' because the internal grammar of Old English says this is
*right*, 'it feels good'. Because English dropped almost all
inflection altogether, some -- lots of -- grammatically logical stuff
got lost, and 'feet' got fossilized, its singular being a *very*
common word. 'Feet' is now inexplicable, except via a long and not
very good explanation like this.
For myself, I'm still left somewhat in the dark. I gather that only
certain classes of OE nouns did this, and of those that did, just
about all of them regularized themselves (via the magic of analogy) to
normal plurals in -s.
So. The short answer is that English was, once upon a time, a highly
inflected language, but has in the meantime lost just about all of its
inflections, and as a consequence, has lost any internal explanation
to a native-speaker of why the plural of 'foot' is 'feet'.
Umlaut is a feature of all the Germanic languages, and would seem to
go back, if not to PIE itself, to something just afterwards.