Student or Learner
Which are the correct tenses here, please?
"We can also use the Present Perfect when we talk/are talking about somehting that happened/ has happened in the past but we don't know
It would be acceptable to use either "talk" or are talking but I would usually use the present simple (talk) because it is a general truth about how we use the present perfect tense.
In a teaching sense I would avoid using "has happened" because we would be using the present perfect to explain the present perfect which would be confusing. (However grammatically there is nothing wrong with what you wrote)