Proto-Indo-European Roots

Root/Stem: *es-
Meanings:  to be
Cognates (44): Greek eimi (I am), esti (is)
Latin sum (I am), es < *ess (thou art), est (is), Oscan ezum (to be), som (I am)
Common Celtic *esmi (I am); > 
Gaulish emmi (I am), esti (is)
Common Germanic *is- (to be), > 
Old English eom (I am), ist (is), Gothic & Old High German ist, Old Frisian is, Old Norse ert
German ist (is), Dutch is, Danish & Norwegian er
Common Baltic *esmi (I am), > 
Old Lithuanian esmi (I am), esti (is), Lithuanian esu (I am), esi (thou art), Prussian asmai (I am), Latvian esmu (I am), esi (thou art), Sudovian esmai (I am), est (is)
Sanskrit & Vedic asmi (I am), asti (is); 
Khowar asum (I am)
Common Anatolian *es- (is), > 
Palaic ash- (to be), Hieroglyphic Hittite asa, sa (to be), Nesian Hittite es-, Luwian as-, Lydian e-
Avestan ahmi (I am), asti (is)
Armenian em (I am) < *esmi
Albanian jam (I am) < *esmi
Common Slavic *esmi (I am), > 
Ukrainian est' (is), Belorussian josc', Serbo-Croatian jesam (I am), jest (is), Slovene je (is), Bulgarian sum (I am), je (is), Czech jsem (I am), jest (is), Slovak som (I am), Polish jesm (I am), jest (is), Sorbian je (is), Russian jest' (is)
Notes: One of the most widespread Indo-European words, this verb is irregular everywhere and evidently was irregular in Proto-Indo-European as well; different forms of it are formed from different stems. 
The parallel stem with the same meaning is *bhu- from which English 'to be' derives. These two stems are sometimes considered to have acted as "active - inactive", ie. *es- meant only animate nouns.