In many languages, the verb form known as the “infinitive” is thought to have originated as the frozen case form of a verbal noun. Below, I’ve listed all the historical examples I can think of right now; what other examples do you know of?
- Greek infinitives in -ai (iénai “to go”, boúlesthai “to want”, etc.) may all come from forms ending with the locative singular suffix *-i or from forms with the dative sg. suffix *-ei. Infinitives in -ein (pémpein “to send”, lipeîn “to have left”, etc.) may also contain a locative suffix -i with a secondary -n added to mark the end of a vowel-final word (cf. the 3pl. endings -ousi/-ousin)
- Latin infinitives in -re/-se (esse “to be”, venire “to come”, etc.) are thought (at least by some) to be from *-si, containing the locative suffix *-i, and the Latin passive infinitive in -(r)i (amari “to be loved”, capi “to be taken”) is traced to a form with dative *-ei.
- The Baltic and Slavic infinitive suffix -ti (Slovene iti, Lithuanian eiti “to go”) is thought to be from *-tei, the locative of a verbal noun formation in -ti-.
- English and Romanian (and probably many other languages) have incorporated a preposition meaning “to” into the basic form of their infinitives: English to do, to be, Romanian a face “to do”, a fi “to be”, etc.
- The Finnish infinitive suffix -(t)a (juosta “to run”, puhua “to speak”) is thought to reflect a form *-tak containing the lative suffix *-k (meaning “(in)to” or similar).
- Germanic infinitives in -an (Old English beran, Gothic bairan “to carry”) are traced back to *-anan, the accusative singular of a verbal noun in *-ono- (or possibly the nominative of this verbal noun, if its gender was neuter).