What does ETRE stand for? 

être

In some of the Romance languages the copula, the equivalent of the verb to be in English, is relatively complex compared to its counterparts in other languages. A copula is a word that links the subject of a sentence with a predicate (a subject complement). Whereas English has one main copula verb (and some languages like Russian mostly express the copula implicitly) some Romance languages have more complex forms. Italian, Portuguese, Spanish, and some other Romance languages have more than one copula verb. Conversely, French and certain others have only one. The development of copula verbs in Romance languages is explained by the fact that these are ultimately derived from three Latin verbs: esse "to be" (ultimately from Proto-Indo-European *h₁es-, as in English is). The verb esse was an irregular, suppletive verb, with some of its forms (e.g. fuī "I was/I have been") taken from the Proto-Indo-European root *bʰuH- meaning "to become" (as in English be). stāre "to stand" or "to stay" (ultimately from Proto-Indo-European *steh₂-, as in English stand and German stehen). sedēre "to sit" (ultimately from Proto-Indo-European *sed-, as in English sit).As the Romance languages developed over time, the three separate Latin verbs became just one or two verbs in the Romance languages.

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