Besides noteworthy instances of inherited verbal inflexion such as the optative in -ā- in Balochi and Judaeo-Tat, marking of TAM categories in New Western Iranian is mostly achieved by way of particles / verbal prefixes. Of central importance is the prefix bi- / be-, which is grammaticalised in New Persian as a marker of the subjunctive and the imperative. These forms differ from the indicative in that the latter is marked with mī-, but the inflexion is essentially identical. We argue that the influence of Persian has had a multiple effect on the TAM system of other Ir. languages. First insofar as prefixes have been borrowed, second insofar as the use of preexisting prefixes has been adjusted to fit the usage of Persian prefixes, and thirdly insofar as different prefixes have coalesced into one under the influence of Persian (such as be-, ba- etc. yielding be- in Bashkardi and other languages). Dialectal variations such as seen in Tat thus carry valuable diachronic information.