Questions & Answers about Я купил дополнительный билет на концерт для друга.
Купил is the perfective past tense of купить, so it presents the action as completed: the ticket was bought.
A native English speaker can think of it as closer to bought in a one-time, finished sense.
By contrast, покупал is imperfective and would usually suggest something like:
- the process of buying,
- a repeated action,
- or background information.
So in this sentence, Я купил... means the speaker is talking about one completed purchase.