My ESL speaker's experience is that, when you're taught about words like "then", they tell you to use it this way: "Then a doctor examined the patients." However, it can be used correctly in another way- the way your teacher showed you. It means the same (determines when- after some other action/event, or at the specific time you're talking about). It's just used in a different place. Still the same thing.I coudn't understand what was the sense of that 'then' in the sentence.