Na afloop bespraken we het toneelstuk op het terras naast het theater.

na afloop literally means “after the end” and functions as an adverbial phrase for “afterwards.” It’s a fixed expression, so you omit de. If you want to specify what ended, you’d say, for example:

• na de afloop van de voorstelling (“after the end of the performance”).

Dutch offers two ways to talk about past actions:

• Perfect tense: we hebben het toneelstuk besproken
• Simple past (preterite): we bespraken het toneelstuk
  In spoken Dutch the perfect is more common, but in writing or storytelling the simple past is perfectly acceptable—and that’s why you see bespraken here.

Dutch follows the “V2” rule in main clauses: the finite verb must occupy the second slot. Since na afloop is the first element, the verb bespraken automatically comes second:

1. na afloop
2. bespraken
3. rest of the sentence…

You actually have two nested location phrases:

• op het terras tells you where the discussion took place.
• naast het theater specifies which terrace—namely, the one next to the theater building.
  You need op for “on” (a surface) and naast for “next to,” so they stay separate.

No. bespreken is a transitive verb, so it directly takes its object (het toneelstuk). You don’t add over. If you want over, switch to the intransitive spreken:

• We bespraken het toneelstuk.
• We spraken over het toneelstuk.