Breakdown of Mi hermano estaba agotado y frustrado, pero aun así terminó el formulario antes de cenar.
Questions & Answers about Mi hermano estaba agotado y frustrado, pero aun así terminó el formulario antes de cenar.
Because estaba (imperfect) sets the background and describes his state as an ongoing condition at that moment.
In this sentence, there are two time layers:
- estaba agotado y frustrado = his condition/state in progress
- terminó el formulario = the completed main event
Spanish often uses the imperfect to describe how someone was feeling while something else happened.
So the sentence structure is basically:
- He was exhausted and frustrated → background
- but even so he finished the form → completed action
If you used estuvo, it would sound more like you are viewing that state as a complete, bounded event, which is less natural here.