Our modern verb to be is actually a conflation of three distinct roots: es-, wes-, and beu-. The modern inflections of to be are a jumble of inflections of these three original roots. And this conflation has been going on for well over 1,200 years.
By the time we reach the Old English period, the es- and wes- forms had already combined into a single verb, wesan. The conjugations of wesan are:
Inflected Infinitive: --
1st Person Singular: eom
2nd Person Singular: eart
3rd Person Singular: is
Plural: sindon, sind, sint
1st Person Singular: wæs
2nd Person Singular: wære
3rd Person Singular: wæs
Note that the modern present and past indicatives are taken from this word: am, are, is, was, and were. The present indicative and present subjunctive of wesen are taken from the es- root, all the other forms are from wes-.
The second verb in Old English meaning to be is beon. It’s inflected as follows:
Inflected Infinitive: to beonne
1st Person Singular: beo
2nd Person Singular: bist
3rd Person Singular: biþ
Note that beon has no past tense, it can only denote the present. As a result, Old English use of beon is thought to sometimes connote a sense of continuous existence, now and into the future. Our modern forms be, been, and being are from this word.
By the beginning of the 13th century, the conflation of beon and wesen was complete and our modern inflections had come into being. Some of the older forms, however, still survive in dialectal speech.
(Source: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2013, by David Wilton