Learn Farsi: قهرمان می‌توانست حریف را در ورزشگاه شکست بدهد. - The champion could defeat the opponent in the stadium.

می‌توانست is the past continuous form of توانستن (to be able to). We use the می‌ prefix here to show a general past ability or a condition ("was able to / had the ability to"), which is how Farsi usually expresses the English "could".

If the sentence is about the past, why is the second verb شکست بدهد in the present subjunctive?

In Farsi, the first verb in the chain takes care of the tense. می‌توانست already tells us this happened in the past. The second verb must be in the present subjunctive, which often starts with the بـ prefix.

How does the present subjunctive work with a compound verb like شکست دادن (to defeat)?

For compound verbs, only the verbal part changes. شکست (defeat) is a noun and stays exactly the same. The verb دادن (to give) takes the subjunctive prefix بـ and the personal ending, becoming بدهد (he/she gives).

Why is there a را after حریف (opponent)?

را is the definite direct object marker. Because the champion is fighting a specific opponent ("the opponent," not just any opponent), we must put را right after حریف.