Proto-Indo-European Roots

Root/Stem: *bhú-
Meanings:  to be
Cognates (33): Greek phuomai (I am becoming) - lit. 'I turn to be'
Latin fuí (I was) - the Aorist-Perfect from the verb esse 'to be'; futurum (the future); Oscan fust (he will be)
French futur (future)
Common Celtic *bei- (to be, to live), *bovet (he was); >
Gaulish bitu (being), Old Irish bi'u (I am), boi (I was), Old Welsh bit (if he were), Middle Breton bezaff (if I were), Scottish & Irish Gaelic bi'm (I am)
Sanskrit bhutis (being)
Avestan bú- (to be, to become), bavainti (he becomes, he appears to be) - a wonderful analogue of Russian "byvat'" with the same meaning
Old High German bim (I am), bist (thou art), Old English béon (to be), Middle English ben (to be);
German bin (I am), Dutch ben
Common Baltic *bú- (to be); >
Lithuanian bu'ti (to be), bu'na (he happens to be), Old Prussian & Latvian  bu't (to be), Sudovian bu'tun, bu't
Common Slavic *byti (to be); >
Ukrainian buti, Serbo-Croatian & Slovene biti, Czech býti, Polish byc', Sorbian bys', Russian byt' (to be), byvat' (to be often somewhere)
Notes: This stem is the other of the couple, the first of which was *es-. It is believed to have been used in Proto-Indo-European as an "inactive" verb, for inanimate things only. Later two stems coincided with each other, and modern languages often have both of them - *bhú- for infinitive forms, *es- for personal ones.
Both remain very widespread within the Indo-European family. Maybe this very word was the first word invented by ancient Indo-Europeans.