är- is the copular verb ‘to be’. It occurs with a nominal, adjectival, or locative predicate to state what the subject is, what it is like, or where it is; it does not independently mean ‘to exist’, a function normally expressed by bār. It also served widely as an auxiliary with participles and other non-finite verb forms.
Although är- originally had a complete conjugational paradigm, its -r- began to disappear during the early medieval period, and many of its finite forms were subsequently replaced by corresponding forms of bol-/ol- (‘to become’) or tur- (‘to stand’). In the modern Turkic languages where bol-/ol- assumed these functions, its older meaning ‘to become’ expanded into the copular domain of ‘to be’, so that the same verb came to express both ‘to become’ and certain uses of ‘to be’.
As an independently conjugated copular verb, är- appears to survive only in Khalaj, Sakha and Northern Altai dialects; elsewhere, only parts of its paradigm remain, chiefly in fossilized, enclitic, or otherwise grammaticalized forms.