Learn Farsi: آنها فرم را پر نکرده بودند.

This sentence uses the compound verb پر کردن (to fill). To say 'had not filled', we take the non-verbal part پر (full), add the negative prefix نـ (na-) to the past participle کرده (done), and finish with the simple past of 'to be' for 'they', which is بودند (they were). Together, they make پر نکرده بودند.

Why does the negative prefix attach to کرده instead of پر?

In Farsi, compound verbs are made of a non-verbal word (like the adjective پر) and a 'light verb' (like کردن). Prefixes like the negative نـ (na-) always attach to the actual verb part. So the negative goes on the verb to make پر نکرده بودند.

Why do we need the word را after فرم?

را (ra) is the specific direct object marker. Because the sentence refers to a specific form ('the form') that they had not filled out, rather than just filling out forms in general, Farsi requires را right after the noun.

Is فرم borrowed from the English word 'form'?

Yes! فرم is a loanword and means exactly the same thing: a document you fill out. Farsi has adopted several European words for modern administrative and everyday concepts.