Learn Spanish (Spain): Fue la casualidad más rara del mundo. Yo leía cuando ella entró. - It was the strangest coincidence in the world. I was reading when she entered.

This sentence perfectly shows how to talk about interrupted actions in the past. We use the imperfect tense for the background action that was already happening in the middle of its progress (leía, meaning "I was reading"). Then, we use the preterite tense for the sudden action that interrupted it (entró, meaning "she entered"). The word cuando (when) is the typical trigger that introduces the interrupting preterite verb.

Why does the first sentence use fue (preterite) instead of era (imperfect)?

It is true that we generally use the imperfect (era) to describe characteristics or scenes in the past. However, here the speaker is summing up a specific, sudden event ("that moment when she walked in"). Because they are evaluating that specific event as a single completed fact rather than setting a continuous background scene, they use the preterite fue.

Why does it say del mundo instead of en el mundo for "in the world"?

When making a superlative statement in Spanish (saying something is the "most" or "least" of a group), you use the preposition de rather than en to define that group. So, to say something is the strangest "in the world," Spanish speakers literally say it is the strangest "of the world" (del mundo).

Could I drop the pronouns yo and ella?