The copula or copulae (the verb or verbs meaning "to be") in all Romance languages largely derive from the Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 verbs "to be" (ultimately from Proto-Indo-European

Proto-Indo-European language

The Proto-Indo-European language is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans...

 *h₁es-, as in English is); "to stand" (ultimately from Proto-Indo-European *steh₂-, as in English stand and German stehen); and "to sit" (ultimately from Proto-Indo-European *sed-, as in English sit). The verb was an irregular, suppletive verb

Suppletion

In linguistics and etymology, suppletion is traditionally understood as the use of one word as the inflected form of another word when the two words are not cognate. For those learning a language, suppletive forms will be seen as "irregular" or even "highly irregular". The term "suppletion" implies...

, with some of its forms (e.g.