Learn Spanish (Latin America): Tras la pelea, todo cambió.

The past perfect connects two past verbs, like if we said "After they habían peleado (had fought), everything changed." In this sentence, the earlier "event" is just a noun phrase (la pelea). Because there's no verb for the first action, we only need one verb for the second action (cambió), which takes the simple preterite.

Can I use después de instead of tras in this sentence?

Yes, absolutely. Tras and después de both mean "after" in this context. Tras is slightly more concise and can sometimes sound a tiny bit more formal or dramatic, but después de la pelea is completely natural and means the exact same thing.

Why use the preterite cambió instead of the imperfect cambiaba?

The preterite is used here to show a completed, definitive shift. The fight acted as a clear trigger that caused the change. If you used the imperfect (todo cambiaba), it would mean "everything was changing" continually in the background, rather than a complete "before and after" shift.

Does todo need an article or noun to mean "everything"?