Tenses in narrative: the historic present
At Β2 you made tenses agree. At Γ1 you choose them: present to make the past vivid, imperfect for the backdrop, aorist for the event.
The historic present
Greek switches to the present in the middle of a past narrative to put the reader on the spot: «Περπατούσα στον δρόμο, και ξαφνικά βλέπω τον Νίκο.» This is not a sequence-of-tenses error — it is an effect, extremely common in speech and journalism.
- Το 1453 πέφτει η Κωνσταντινούπολη.In 1453 Constantinople falls.
Background and foreground
In narrative, the imperfect lays down the setting and lasting states; the aorist moves the action forward. Swapping one for the other does not make the sentence wrong — it moves what the reader perceives as the main event.