Online Etymology Dictionary

be (v.)

Old English beon, beom, bion "be, exist, come to be, become, happen," from Proto-Germanic *biju- "I am, I will be." This "b-root" is from PIE root *bheue- "to be, exist, grow, come into being," and in addition to the words in English it yielded German present first and second person singular (bin, bist, from Old High German bim "I am," bist "thou art"), Latin perfective tenses of esse (fui "I was," etc.), Old Church Slavonic byti "be," Greek phu- "become," Old Irish bi'u "I am," Lithuanian bu'ti "to be," Russian byt' "to be," etc. It also is behind Sanskrit bhavah "becoming," bhavati "becomes, happens," bhumih "earth, world."

The modern verb to be in its entirety represents the merger of two once-distinct verbs, the "b-root" represented by be and the am/was verb, which was itself a conglomerate. Roger Lass ("Old English") describes the verb as "a collection of semantically related paradigm fragments," while Weekley calls it "an accidental conglomeration from the different Old English dial[ect]s." It is the most irregular verb in Modern English and the most common. Collective in all Germanic languages, it has eight different forms in Modern English:

BE (infinitive, subjunctive, imperative)
AM (present 1st person singular)
ARE (present 2nd person singular and all plural)
IS (present 3rd person singular)
WAS (past 1st and 3rd persons singular)
WERE (past 2nd person singular, all plural; subjunctive)
BEING (progressive & present participle; gerund)
BEEN (perfect participle).

The paradigm in Old English was:

	SING.	PL.
1st pres.	ic eom ic beo	we sind(on) we beoð
2nd pres.	þu eart þu bist	ge sind(on) ge beoð
3rd pres.	he is he bið	hie sind(on) hie beoð
1st pret.	ic wæs	we wæron
2nd pret.	þu wære	ge waeron
3rd pret.	heo wæs	hie wæron
1st pret. subj.	ic wære	we wæren
2nd pret. subj.	þu wære	ge wæren
3rd pret. subj.	Egcferð wære	hie wæren

The "b-root" had no past tense in Old English, but often served as future tense of am/was. In 13c. it took the place of the infinitive, participle and imperative forms of am/was. Later its plural forms (we beth, ye ben, they be) became standard in Middle English and it made inroads into the singular (I be, thou beest, he beth), but forms of are claimed this turf in the 1500s and replaced be in the plural. For the origin and evolution of the am/was branches of this tangle, see am and was.

That but this blow Might be the be all, and the end all. ["Macbeth" I.vii.5]