Three phases may be distinguished in the development of Persian: Old, Middle, and Modern. Old Persian is recorded in the southwest in cuneiform inscriptions of the Persian kings of the Achaemenid dynasty (circa 550-330 BC), notably Darius I and Xerxes I. Old Persian and Avestan have close affinity with Sanskrit, and, like Sanskrit and all ancient Indo-European languages, are highly inflected languages. Old Persian phonetics represented three pair of monophthongs: long and short a, i, u, diphthongs ai, au, 23 consonant phonemes. The morphology shows seven or eight cases, three numbers, three genders, verbs had very complicated structure. The vocabulary borrowed plenty of words from other ancient Iranian languages, and alos from non-Indo-European tongues, e.g. from Aramaic. As a whole, we can say that Old Persian was the beginnig of those significant processes which led the language to its modern stage, quite different from the ancient one.
The language of the Sassanian Empire (AD 224-641) was Middle Persian, often called Pahlavi (a term more strictly reserved for a form of the language used in certain Zoroastrian writings). Middle Persian has a simpler grammar than Old Persian and was usually written in an ambiguous script with multivalent letters, adopted from Aramaic; it declined after the Arab conquest in the 7th century. Although much of the Middle Persian literature was translated into Arabic, the bulk of its writings was lost during Islamic times. In Middle Persian times phonetics changes greatly: e.g., Indo-European g'h which became z in Old Persian now turned into d; s > h, kw > sp > s , etc. Much influence it suffered from Parthian and other neighbor languages, and certainly from Arabic. The morphology now becomes completely analytic, loses genders and cases, many verbal forms.
Modern Persian had developed by the 9th century. It is a continuation
of an area-wide standard language that had considerable Parthian and Middle
Persian elements, with additional influences from other Iranian languages.
Written in Perso-Arabic script (an expanded version of Arabic script),
it has been the official and cultural language of Persia since it first
appeared. Its grammar is simpler than that of Middle Persian, and it has
absorbed a vast Arabic vocabulary. No gender and noun case, nouns have
only categories of definiteness and number. The definite article -
is used postpositively (after the noun). All verbs are conjugated
following the same type, and came be simple and complex.