Learn Farsi: آنها درباره‌ی این امپراتوری باستانی در کتاب خوانده بودند. - They had read about this ancient empire in the book.

The past perfect ('had read') is made of two parts: the past participle of the main verb (خوانده, meaning 'read') and the simple past of the helper verb بودن to match 'they' (بودند, meaning 'they were'). Together, خوانده بودند means 'they had read'.

Why does the word for 'about' (درباره) have a ی at the end here?

The preposition درباره (darbâre) requires an ezâfe to connect it to the noun that follows it. Since درباره ends in a silent 'e' sound (written with the letter ه), we must add the consonant ی (y) to connect it, resulting in درباره‌ی (darbâre-ye).

Is there another ezâfe hiding in امپراتوری باستانی?

Yes! To connect the noun امپراتوری (empire) to its adjective باستانی (ancient), you need an ezâfe. Because امپراتوری ends in the vowel 'i', the ezâfe is pronounced as a 'ye' sound: emperâturi-ye bâstâni. It isn't written out as an extra letter here, but you must say it when speaking.

Why does در کتاب (in the book) come right before the verb?

Farsi sentences generally put the verb at the very end. The two phrases درباره‌ی این امپراتوری باستانی (about this ancient empire) and در کتاب (in the book) can technically swap places, but putting right before the verb feels natural and highlights exactly where they did the reading.