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.

Because agotado and frustrado describe how the brother felt, not what caused the feeling.

• agotado = exhausted
• frustrado = frustrated

These are past participle forms used as adjectives.

By contrast:

• agotador = exhausting
• frustrante = frustrating

Those describe something that causes the feeling.

So:

• Mi hermano estaba agotado y frustrado = my brother felt exhausted and frustrated
• El formulario era agotador y frustrante = the form was exhausting and frustrating

They agree with mi hermano, which is masculine singular.

In Spanish, adjectives usually match the noun they describe in gender and number.

Here:

• hermano = masculine singular
• so the adjectives are agotado and frustrado

If it were mi hermana, you would say:

• Mi hermana estaba agotada y frustrada

If it were plural:

• Mis hermanos estaban agotados y frustrados
• Mis hermanas estaban agotadas y frustradas

Because terminó is preterite, and the speaker is presenting the action as a completed event.

• terminó = he finished
• terminaba = he was finishing / used to finish / was in the process of finishing

In this sentence, the important point is that he successfully completed the form before dinner. That is why the preterite is the natural choice.

So the contrast is:

• estaba = ongoing background state
• terminó = completed action

This imperfect + preterite combination is very common in Spanish storytelling.

Because Spanish usually leaves out subject pronouns when the verb already makes the subject clear.

Here, terminó clearly means he/she/it finished, and the previous noun mi hermano already tells us who it is.

So Mi hermano ... terminó... is completely natural.

You could say él terminó, but that would usually add emphasis or contrast, for example:

• Mi hermano estaba agotado, pero él terminó el formulario; yo no.

In normal neutral Spanish, the pronoun is often omitted.

Aun así means something like:

• even so
• all the same
• nevertheless

It shows contrast: despite being exhausted and frustrated, he still finished the form.

In this sentence:

• pero = but
• aun así = even so / nevertheless

Together, pero aun así strongly marks the contrast. It is not unusual to use both. It gives a sense like:

• but even so
• but still

So the sentence emphasizes that finishing the form happened despite the difficulty.

In modern standard Spanish, aun without an accent is generally used when it means even, and aún with an accent usually means still/yet.

In aun así, the meaning is closer to even so, so aun without the accent is the standard spelling.

Examples:

• aun así = even so
• aún no ha llegado = he still hasn’t arrived

You may sometimes see variation in older texts or informal writing, but aun así is the standard form here.

Spanish often uses the definite article more than English does.

Here, el formulario refers to a specific form that both speaker and listener can identify from context.

So Spanish naturally says:

• terminó el formulario

Even if English might sometimes say finished the form or just finished a form depending on context, Spanish commonly includes the article when referring to a particular known item.

If it were an unspecified form, you might say:

• terminó un formulario

But in this sentence, el formulario sounds like a specific one.

Because after antes de, Spanish often uses an infinitive when talking about an action.

• antes de cenar = before having dinner / before eating dinner

This is very natural and common.

You can also use a noun phrase:

• antes de la cena = before dinner

Both can be correct, but they are slightly different in structure:

• antes de cenar focuses on the action of eating dinner
• antes de la cena focuses on the meal as an event/time

In many everyday situations, antes de cenar sounds especially natural.

Yes, but it means something slightly different and needs a different structure.

• antes de cenar = before eating dinner
• antes de que cenara = before he/I/someone ate dinner

After antes de que, Spanish uses a conjugated verb instead of an infinitive, and the subject is usually more explicit or implied by context.

In your sentence, antes de cenar is simpler and more natural because the subject is the same person: the brother finished the form before he had dinner.

Because it is the topic and subject, and Spanish often starts with the subject when introducing who the sentence is about.

The basic order here is very straightforward:

• Mi hermano = subject
• estaba agotado y frustrado = description of his state
• pero aun así terminó el formulario = contrasting action
• antes de cenar = time expression

Spanish word order is flexible, so other arrangements are possible for emphasis, but this version is the most neutral and natural.

For example, you could say:

• Agotado y frustrado, mi hermano terminó el formulario antes de cenar.

That sounds more literary or more emphatic.

Yes, that is the main idea.

Spanish normally uses estar for states or conditions that are seen as temporary, changeable, or true at a particular moment:

• estaba agotado
• estaba frustrado

These describe how he felt at that time.

Using ser here would sound wrong in normal Spanish because ser frustrado or ser agotado does not express temporary emotional/physical states in the same way.

So with emotions, physical conditions, and momentary states, estar is usually the correct verb.

Because pero joins two contrasting clauses, and Spanish normally places a comma before it in this kind of sentence.

Here the two parts are:

• Mi hermano estaba agotado y frustrado
• pero aun así terminó el formulario antes de cenar

The comma helps show the contrast clearly and makes the sentence easier to read.

This is similar to English, where we also often put a comma before but when joining two full clauses.