Learn Farsi: دشمن نمی‌توانست گروگان را نکشد.

This sentence uses a negative modal verb, نمی‌توانست (could not), followed by a negative subjunctive main verb, نکشد (not kill). In Farsi, combining these two negatives creates a strong sense of inevitability or absolute obligation, translating literally to 'could not not kill' or 'had to kill'.

Why is the second verb نکشد in the present subjunctive if the sentence is about the past ('couldn't')?

In Farsi, even when a modal verb is used in the past tense like نمی‌توانست, the following main verb takes the present subjunctive. The past tense of the modal establishes the timeframe for the whole sentence, so نکشد simply expresses the dependent action rather than its own tense.

Does the stress fall on a different syllable in نکشد because it is negative?

Yes. In negative Farsi verbs, the stress strongly shifts to the negative prefix. So in نکشد, you emphasize the first syllable, which helps clearly signal the double-negative grammar in fast spoken Farsi.

Why is the direct object گروگان را placed between the two parts of the verb phrase?

While Farsi is a Subject-Object-Verb language, sentence structure with modal verbs is slightly flexible. Placing the specific object گروگان and its marker را between the modal نمی‌توانست and the main verb نکشد is very natural and ensures the main action verb lands at the absolute end of the sentence.