Learn Farsi: آنها نمی‌توانستند از دره به آن صخره صعود کنند. - They couldn't climb from the valley to that rock.

In Farsi, the verb توانستن (to be able to) always takes the present subjunctive for the main action, regardless of whether the capability was in the past, present, or future. So, the helping verb نمی‌توانستند changes to show the past tense, but the action verb صعود کنند remains in the present subjunctive.

Why is 'they couldn't' translated using the past continuous نمی‌توانستند instead of the simple past?

To express a general past ability or ongoing possibility (meaning 'they used to be able to' or 'they generally were not able to'), Farsi uses the past continuous form, which includes the می prefix. If you used the simple past نتوانستند, it would mean they tried and failed at one specific, completed moment in the past.

If I want to say 'they could not climb', why does the negative prefix نـ go on می‌توانستند and not on the climbing verb?

When using a modal verb like توانستن with a subjunctive verb, you negate the modal verb to show a lack of ability. So نمی‌توانستند means 'they did not have the ability', while the main action صعود کنند remains affirmative.

Why doesn't the subjunctive verb صعود کنند have the usual بـ prefix?

The verb صعود کردن (to climb or ascend) is a compound verb. When forming the subjunctive of a compound verb with کردن, the prefix is usually dropped in modern Farsi because the non-verbal part () already makes the meaning clear enough. You just apply the personal ending to the verb stem, which gives us .