Learn Farsi: مکاتبات این تشکیلات خیلی پیچیده است. - The correspondence of this organization is very complicated.

They both use the Arabic plural suffix -ât, but they function a bit differently here. mokâtebât is the plural of mokâtebeh (letter/communication). tashkilât, while originally the plural of tashkil (formation), is commonly used in Farsi as a collective singular noun meaning an entire organization or establishment.

If tashkilât has a plural ending, why does it use the singular word for this (in) instead of these (in-hâ)?

Because tashkilât acts as a collective noun representing a single entity (the organization), it takes singular modifiers like in (this). You will see this with other -ât words too, like etellâ'ât (information), which is also treated as a singular concept despite its plural shape.

The subject of the sentence is mokâtebât (plural). Why is the verb singular (ast) instead of plural (and)?

In Farsi, plural inanimate subjects (things that are not human) almost always take a singular verb. Since correspondence is not human, the verb ast (is) remains singular, even though the noun is technically plural.

How does the word pichideh come to mean complicated?