Learn Farsi: نتوانستیم ردیابی‌اش کنیم، بنابراین تعقیبش نکردیم. - We could not track him, therefore we did not pursue him.

The pronoun "him" is expressed by the enclitic -ash. As this lesson shows, instead of using the independent pronoun and object marker (u râ), Farsi frequently attaches the object enclitic directly to the noun portion of a compound verb. So, instead of u râ rad-yâbi konim, you get rad-yâbi-ash konim.

Why is the -ash enclitic spelled with an alef in rad-yâbi-ash (ردیابی‌اش) but not in ta'qib-ash (تعقیبش)?

It depends on the final sound of the noun it attaches to. Rad-yâbi ends in a vowel (the ee sound). To attach the -ash ending without making two vowels clash, an alef (ا) is inserted in writing as a bridge. Ta'qib ends in a consonant (b), so the enclitic just attaches directly to the final letter.

Why is the verb konim in the subjunctive mood when the sentence is clearly happening in the past?

The verb tavânestan (to be able to) always forces the verb that follows it into the subjunctive mood, no matter the overall tense of the sentence. The literal structure here is "We were not able that we track him." The main verb natavânestim establishes the past tense, while konim must take the subjunctive.

If the first half uses the subjunctive konim, why does the second half use the simple past nakardim?

The word banâbar-in (therefore) starts a new, independent clause. The second action ("we did not pursue him") is a standalone fact that is no longer governed by (we could not). Therefore, it drops the subjunctive and returns to the simple past tense to match the timeframe of the story.