Indo-European copula
A feature common to all Indo-European languages is the presence of a verb corresponding to the English verb to be. Though in some languages it is vestigial, it is present nonetheless in atrophied forms or derivatives.
This verb has two basic meanings. In a less marked context it is a simple copula (I'm tired; That's a shame!), a function which in non-Indo-European languages can be expressed quite differently. In a more heavily marked context it expresses existence (I think therefore I am); the dividing line between these is not always easy to draw. Some languages have shared these functions between several verbs: Irish, Spanish and Persian all have multiple equivalents of to be, making a variety of distinctions. Many Indo-European languages also use the conjugations of the verb "to be" as an auxiliary for the formation of compound (periphrastic) tenses (I'm working; I was bitten). Other functions vary from language to language. For example, although in its basic meanings, to be is a stative verb, English puts it to work as a dynamic verb in fixed collocations (You are being very annoying).
The copula is the most irregular verb in many Indo-European languages. This is partly because it is more frequently used than any other, and partly because Proto-Indo-European offered more than one verb suitable for use in these functions, with the result that the daughter languages, in different ways, have tended to form suppletive verb paradigms. This article describes the way in which the irregular forms have developed from a series of roots.
The root *h1es- was certainly already a copula in Proto-Indo-European. The e-grade (see Indo-European ablaut) is found in such forms as English is, Irish is, German ist, Latin est, Sanskrit asti, while the zero grade produces forms beginning with /s/, like German sind, Latin sumus, Vedic Sanskrit smas, etc. In PIE, *h1es- was an athematic verb in -mi; that is, the first person singular was *h1esmi; this inflection survives in English am, Persian am, Sanskrit asmi, Old Church Slavonic есмь (esmĭ), etc.
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