Learn Farsi: آنها فکر می‌کردند که جاودانه است، اما شکستش دادند. - They thought he was immortal, but they defeated him.

Instead of using the independent pronoun and object marker (u râ shekast dâdand), it is very natural in Farsi to attach the object enclitic -ash (him/her/it) directly to the noun part of the compound verb. So shekast dâdan (to defeat) becomes shekast-ash dâdand. This structure is heavily favored in both spoken and written Farsi because it makes the sentence flow more smoothly.

If the enclitic -ash means 'him', could I attach it to the end of the verb instead, like shekast dâdand-ash?

No. When adding a pronominal enclitic to a compound verb (which is made of a noun or adjective plus a light verb like dâdan), the enclitic almost always attaches to the non-verb component. Attaching it to the light verb itself sounds unnatural in this structure.

Why does the first clause use the past continuous (fekr mi-kardand) for 'they thought'?

In Farsi, verbs describing mental states, opinions, or feelings in the past (like thinking, knowing, or wanting) usually take the continuous prefix mi-. This highlights that the belief was an ongoing state leading up to the defeat, even though English simply translates it as 'they thought'.

Why does the sentence use ast (is) for 'he was immortal' when the rest of the sentence is in the past tense?

Farsi often uses direct speech rules for reported thoughts. Imagine what was in their heads at the time: 'He is immortal.' Farsi preserves that present-tense perspective with (is). English grammar forces a tense shift to match 'they thought', which is why the translation says 'he immortal'.