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At their earliest stage of attestation, most Indo-European (IE) languages show either a single infinitive suffix or a range of verbal noun formations distributed in lexically unpredictable fashion. To the first category belong Greek, Latin, Germanic, and Balto-Slavic, whereas Old Irish and Vedic are famously of the second type, which must be regarded as an archaism in view of the absence of a securely reconstructible infinitive for Proto-Indo-European (PIE). The grammaticalization of the infinitive much thus have taken place separately in each branch; this was demonstrably a late prehistoric development in Anatolian, where Hittite has both -anna and -wanzi; and Italic, where Latin -re, es-se, vel-le (< *-si) contrast with Sabellic -om.
This study examines the evolution of infinitive formation in Tocharian and Iranian, two branches often overlooked in previous diachronic treatments. In Tocharian, both languages A and B share the suffix -tsi, but with a crucial difference: with very few exceptions, the infinitive in Tocharian A is formed to the present stem, in Tocharian B to the subjunctive stem. It is argued that this situation can only have developed from an earlier stage in which a verbal noun in -tsi could be formed to both stems, which in turn favors an origin for this suffix in PIE *-dhyey, originally locative of a verbal noun in *-dhyo- (< *-dhh1-yo-?) standing beside dative *-dhyōy in Vedic -dhyai, Old Avestan -diiāi.
In Iranian, the diversity of infinitives inherited from Proto-Indo-Iranian and still preserved in Avestan has been variously reduced in the other languages, for instance to Old Persian /-tanai/ (> Middle, Modern Persian past stem + -tan, -dan), Khotanese past stem + –ie beside present stem + -i/-ä, or Ossetic present stem + -un (Digor) / -yn (Iron). The grammaticalization pathways of these formations are investigated, and an attempt is made to identify the factors influencing the selection of particular markers in the Middle and Modern Iranian languages.