Learn Spanish (Latin America): El choque que ocurrió ayer arruinó el tráfico. - The crash that happened yesterday ruined the traffic.

It usually refers to a physical collision like a car crash, but it can also mean a shock or clash in other contexts, like an electric shock (choque eléctrico) or a culture clash (choque cultural). Here, combined with tráfico, it definitely means a car crash.

Why do we use the preterite forms ocurrió and arruinó instead of the imperfect tense?

Both verbs describe actions that are viewed as entirely completed events in the past. The word ayer (yesterday) pinpoints exactly when the crash happened, making the preterite ocurrió necessary. Similarly, the ruining of the traffic is treated as a completed past event (arruinó).

Could I use quien instead of que here?

No, quien is only used for people, meaning who or whom. Since el choque is a thing, you must use the relative pronoun que, which means that or which in this context.

Is arruinó an exact match for the English word ruined?

Yes! The verb arruinar means to ruin or destroy. It is a direct cognate and works very similarly to English, so you can use it when something spoils a plan, an object, or in this case, the flow of traffic.