Perhaps the most significant difference between Ancient and Modern Greek is replacement of the infinitive by complementation in the subjunctive. The infinitive became very restricted in use by the tenth century and was finally lost by the 16th century.
Why was the infinitive lost? It may be that the Greek infinitival system, which included 13 different infinitives, was simply too complicated to last.
The MG subjunctive complement is introduced by να derived from Ancien Greek ἵνα. Trypanis showed that the accent moved on the basis of metrical evidence from post-classical Greek (Glotta 38 312.) The initial unaccented vowel was then lost. Julian Mendes Dosuna thinks it probably had a completely unaccented form.
The comprehensive treatments of the loss of the infinitive are the following:
- Morphology and Universals in Syntactic Change: Evidence from Medieval and Modern Greek. Ph.D. Dissertation, Harvard (1978). Printed and distributed by the Indiana University Linguistics Club. Pp. 293 + x. Expanded and updated version published by Garland Publishers (Outstanding Dissertations in Linguistics Series), 1990. Pp. 314 + xviii.
- The Synchrony and Diachrony of the Balkan Infinitive: A Study in Areal, General, and Historical Linguistics. Cambridge University Press, Cambridge Studies in Linguistics, Supplementary Series, 1983. Pp. 341 + xiv.