être

• Old French ester, from Latin stō (“stand, be situated”), supplied imperfect forms that once originally supplied by forms in ier-/er- (also as the future stems due to analogy with future forms containing -r-) in Old French. The forms with initial ét- derive from this verb.
• Old French estre, from Latin sum (“be”), was a highly irregular verb also in Latin, and derives from Proto-Indo-European *bʰuH- for the forms beginning with f- (see more at fuī), and *h₁es- for the remainder.
  • Instead of forming expected *esse from Latin esse (the infinitive form of sum), it has been regularized by adding third conjugation ending -ere, resulting on *essere. Many others also follow this development.
  • The first-person singular indicative present form suis seems to be derived from *suī, a modified form of fuī by replacing f- with s-.
  • The first-person plural indicative present form sommes is conflated from Latin sumus and Vulgar Latin *esmus (compare Old French esmes, modified from estis (“second-plural indicative present form”), related to êtes).
• Some forms of estre are from conflation of Vulgar Latin *essere and sedēre (infinitive of sedeō (“I sit”)), later sounds similar and resulted on *seēre. The forms with initial soi- and ser- derive from these verbs.
  • The present subjunctive forms are from sedeō and develop independently, but -e- in the third-person singular is replaced by -t in Old French (similar to Modern French avoir), but later -e- is completely removed from singular forms. The deletion of -e- is also similarly once happened in the third-person singular imperfect form in Old French (-oie, -oies, -oit). This is likely due to the subjunctive imperfect suffixes -ât/-ît/-ût.

Cognates in regional languages in France include Bourguignon and Franco-Provençal étre; Champenois ètre; Norman and Gallo ête; Picard ète; Occitan èsser; Corsican esse. Other Romance cognates include Italian essere, and some forms of Portuguese and Spanish ser.