Questions & Answers about A propósito, no vas a creer lo que pasó.
Does a propósito mean 'by the way' or 'speaking of which' here, and can I use por cierto instead?
It can mean either! Both a propósito and por cierto are digressive markers used to introduce a new or tangentially related topic. In this sentence, you could absolutely swap it for Por cierto, no vas a creer... and the meaning would be exactly the same. Just remember that in other contexts, a propósito can also mean 'on purpose,' whereas por cierto does not.
Why do we need lo que instead of just qué to say 'what happened'?
We use lo que (meaning 'that which' or 'the thing that') because 'what' isn't functioning as a question word here. Instead, it's a relative pronoun acting as the concept or object that the person is not going to believe. You only use qué for direct and indirect questions, or exclamations.
Why is pasó in the preterite tense here rather than the imperfect?
The speaker is building up suspense about a specific, completed event. Using the preterite pasó treats the event as a closed action. If you used the imperfect (lo que pasaba), it would translate to 'what was happening' or 'what used to happen,' which loses the dramatic punch of a single piece of shocking news.
Could I use other verbs like ocurrir or suceder instead of pasar?