Middle Persian different from Pahlavi
sound changes, such as
the dropping of unstressed initial vowels
the epenthesis of vowels in initial consonant clusters
the loss of -g when word final
change of initial w- to either b- or gw- → g-
changes in the verbal system, notably the loss of distinctive subjunctive and optative forms, and the increasing use of verbal prefixes to express verbal moods
a transition from split ergative back to consistent nominative-accusative morphosyntactic alignment
changes in the vocabulary, particularly the establishment of a superstratum or adstratum of Arabic loanwords replacing many Aramaic loans and native terms.
the substitution of the Pahlavi script for the Arabic scriptArsacid miθr > Sassanid mihr 'Mithra, contract' (Pahlavi mtr', Manichaean myhr).The phoneme /ɣ/ (as opposed to the late allophone of /g/) is rare and occurs almost only in learned borrowings from Avestan and Parthian, e.g. moγ (Pahlavi mgw or mwg 'Magian'), maγ (Pahlavi mγ) 'hole, pit'.The sound /ʒ/ may also have functioned as a marginal phoneme in borrowings as well.The phoneme /l/ was still relatively rare as well, especially so in Manichaean texts, mostly resulting from Proto-Iranian *rd, *rz and, more rarely, *r. It also occurred in the combination /hl/, which was a reflex of Old Persian /rθ/ and /rs/ (cf. the words 'Pahlavi' and 'Parthian').The sound /xw/ may be viewed as a phoneme or merely as a combination of /x/ and /w/.
Usually /x/, /xw/ and /ɣ/ are considered to have been velar; a less common view is that /x/ and /ɣ/ were uvular instead.Finally, it may be pointed out that most scholars consider the phoneme /w/ as being still a labial approximant, but a few regard it as a voiced labial fricative /v/.The initial clusters of /s/ and a stop (/sp-/, /st-/, /sk-/) had acquired a prosthetic vowel /i/ by the time of the Manichaean Middle Persian texts: istāyišn (ՙst՚yšn) 'praise' vs Pahlavi stāyišn (ՙst՚dšn') 'praise'.Prosody
Stress was on the last syllable. That was due to the fact that any Old Persian post-stress syllables had been apocopated:
Old Persian pati 'at' > Middle Persian pad
Old Persian martiya- 'man' > Middle Persian mard
Old Persian martiyā́nām 'man' (genitive-dative plural) > Middle Persian mardānIt has been suggested that words such as anīy 'other' (Pahlavi spelling AHRN, AHRNyd, Manichaean ՚ny) and mahīy 'bigger' (Manichaean mhy) may have been exceptionally stressed on the first syllable, since the last one was apocopated already in the course of the Middle Persian period: the later forms are an (Manichaean ՚n), and meh (Pahlavi ms and Manichaean myh); indeed, some scholars have reconstructed them as monosyllabic any, mahy even for Middle Persian.