Origins
The verb 'to be' is the most frequently used and most irregular verb in English, and its irregularity is not a historical accident but a fossil record of three separate Proto-Indo-European verbs that were pressed into a single paradigm. The infinitive 'be' and the forms 'been,' 'being' descend from Old English 'bēon,' from Proto-Germanic *beuną, from PIE *bʰuH- (to become, to grow, to appear). The forms 'am' and 'is' descend from PIE *h₁es- (to be, to exist). The forms 'was' and 'were' descend from PIE *h₂wes- (to dwell, to stay, to remain). English speakers conjugate three etymologically unrelated verbs every time they move through the paradigm of 'to be.'
The PIE root *bʰuH- originally meant 'to become, to grow, to come into being' — a dynamic, processual sense rather than the static 'existence' that 'be' now typically expresses. This dynamic meaning is preserved in many of its reflexes. Latin 'fuī' (I was, I have been) is the perfect tense of 'esse' (to be), supplied by *bʰuH- because *h₁es- lacked a perfect form — so even Latin fused two PIE verbs into one paradigm. Greek 'phúein' (φύειν, to bring forth, to produce, to grow) gave rise to 'phúsis' (φύσις, nature — literally 'growth, the process of becoming'), whence English 'physics,' 'physical,' and 'physiology.' Latin 'futūrus' (about to be, going to be) — the future participle of 'esse,' also from *bʰuH- — gave English 'future.' Sanskrit 'bhávati' (becomes, exists, comes to be) is the direct cognate.
The three-root structure of 'to be' is not unique to English. German shows the same pattern: 'bin' (am) from *bʰuH-, 'ist' (is) from *h₁es-, 'war' (was) from *h₂wes-. Dutch, Scandinavian languages, and Gothic all exhibit similar suppletive paradigms, indicating that the merger of these three verbs occurred in Proto-Germanic or even earlier.
Old English Period
The auxiliary uses of 'be' are equally ancient in their origins but innovative in their scope. 'Be' forms the progressive aspect ('I am running') and the passive voice ('the letter was written'). The progressive use — expressing an ongoing action — developed from Old English constructions with 'bēon' + present participle, and became fully grammaticalized during the Early Modern period. The passive use — expressing that the subject receives the action — is shared across the Germanic languages and was already established in Old English.
The philosophical weight of the verb 'to be' has made it a central concern of Western metaphysics from Parmenides onward. The fact that English uses a single verb for existence ('I am'), identity ('I am John'), predication ('the sky is blue'), and location ('the book is on the table') has shaped how English speakers conceptualize these as related phenomena. Many languages use different verbs for these functions — Spanish distinguishes 'ser' (essential being) from 'estar' (temporary state), and languages like Chinese have no copula at all for adjectival predication.