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: "to be" (ultimately from Proto-Indo-European , as in English ). The verb was an irregular, suppletive verb, with some of its forms (e.g. "I was/I have been") taken from the Proto-Indo-European root meaning "to become" (as in English ). "to stand" or "to stay" (ultimately from Proto-Indo-European , as in English and German ). "to sit" (ultimately from Proto-Indo-European , as in English ). As the Romance languages developed over time, the three separate Latin verbs became just one or two verbs in the Romance languages. The reduction of three separate verbs into just one or two appears to have occurred as follows: The irregular infinitive was remodeled into . and forms sounded similar in Latin once the latter reduced to , and sounded even more similar after stress shifted in Spanish infinitives to the penultimate vowel. As a result, parts of the conjugations of erstwhile were subject to being integrated into conjugation paradigms associated with , eventually . itself remained a separate verb, but (later ) and were similar in some meanings, so that, especially in the Western Romance languages, evolved into a second copula, with a meaning of "to be (temporarily or incidentally)"; was then narrowed to mean "to be (permanently or essentially)". The development of two copular verbs in this manner occurred most completely in Spanish, Portuguese and Catalan. In other languages, most usages of English "to be" are still translated by : In Italian, the infinitive essere continues Latin as existential 'to be', while stare has the primary meaning "to stay" and is used as a copula only in a few situations: to express one's state of physical health (sto bene "I am well"); to form progressive aspects (sto parlando "I am speaking");