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.

The reduction of three separate verbs into just one or two appears to have occurred as follows:

• The irregular infinitive esse was remodeled into *essere.
• *essere and sedēre forms sounded similar in Latin once the latter reduced to *seēre, and sounded even more similar after stress shifted in Spanish infinitives to the penultimate vowel. As a result, parts of the conjugations of erstwhile sedēre were subject to being integrated into conjugation paradigms associated with *essere, eventually ser.
• stāre itself remained a separate verb, but stāre (later *istāre) and *essere were similar in some meanings, so that, especially in the Western Romance languages, stāre evolved into a second copula, with a meaning of "to be (temporarily or incidentally)"; *essere 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 *essere:

• In Italian, the infinitive essere continues Latin esse 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"); and (especially in the south of Italy) with the meaning of "to be located", although a distinction can be expressed in most varieties of Italian: è in cucina 'it's in the kitchen (where it usually is)' versus sta in cucina 'it's in the kitchen (where it isn't usually located)'.
• In Old French, the verb ester < stāre maintained the Proto-Romance meaning of "to stand, stay, stop". In modern French, this verb has almost totally disappeared (see below for the one exception), although the derivative verb of rester ("to remain") exists, and some parts of the conjugation of ester have become incorporated into être "to be" < *essere. As a result of this complex evolution, even though French has a single verb for "to be" (être), its conjugation is highly irregular.