Etymology

From Proto-Slavic *byti, from Proto-Balto-Slavic *bʔutei, from Proto-Indo-European *bʰuh₂- (see Proto-Indo-European *bʰew-). Baltic cognates include Lithuanian būti and Latvian būt. Other Indo-European cognates include Latin fuī ‎(“I was”), Ancient Greek φύω ‎(phúō), Sanskrit भवति ‎(bhávati, “to become”), and Persian بودن ‎(budan, “to be”).

Suppletive present forms are also inherited; ѥсмь ‎(jesmĭ), from Proto-Slavic *jesmь, ultimately from Proto-Indo-European *h₁es-. Cognates include Old English wesan (eom, eart, is), Old Norse em, est, es, Gothic 𐌹𐌼 ‎(im), 𐌹𐍃 ‎(is), 𐌹𐍃𐍄 ‎(ist), Avestan ahmi [script needed] (Persian بودن ‎(budan) and است ‎(ast)), Greek εἰμί, εἶ ‎(eî), ἐστί, Latin esse (sum, es, est, sumus, estis, sunt), (from which French être, Italian essere, Spanish ser), Lithuanian esu, Albanian jam.