îs

Where it comes from

Inherited from Latin sum and Latin sunt. Both passed through Vulgar Latin as *su, with the final -u being dropped centuries later through regular sound change, becoming *s, after which a prothetic î- was inserted to ease pronunciation. Unlike Western Romance, Common Romanian underwent another final consonant deletion alongside the first, universal one, by which all remaining instances of final -n (primarily in third-person plural 3rd and 4th conjugation endings) also got deleted. As both words converged phonetically in Proto-Romanian, the distinction between the first-person singular and third-person plural forms of fi vanished, as was also the case with all 3rd and 4th conjugation verbs. For a similar development of prothetic î- being inserted at the beginning of one-consonant-words, cf. the development of îl, and for a similar case of the first-person singular and third-person plural forms of "to be" being the same, cf. Italian sono.