Mi hermano acabó quedándose dormido en el sofá después de estudiar física toda la tarde.

It means ended up falling asleep or ended up dozing off.

The key pattern is acabar + gerundio, which often means to end up doing something. It suggests that this was the final result of a situation, not necessarily the original intention.

So:

• acabó = he ended up
• quedándose dormido = falling asleep

Together, Mi hermano acabó quedándose dormido gives the idea that after everything else, that was what happened in the end.

Both are possible, but they are not exactly the same.

• se quedó dormido = he fell asleep
• acabó quedándose dormido = he ended up falling asleep

The version with acabó + gerundio adds a nuance of result or outcome. It can imply something like:

• after a while
• because of the circumstances
• almost unintentionally

So in this sentence, it sounds a bit more like: after studying physics all afternoon, the final result was that he drifted off on the sofa.

Because quedando is a gerund (the -ing form), and in Spanish object/reflexive pronouns can be attached to gerunds.

So:

• acabó quedándose dormido
• se acabó quedando dormido

Both are correct.

In this sentence, attaching the pronoun to the gerund is very natural. Spanish often allows both positions:

• before the conjugated verb
• attached to the gerund or infinitive

Quedarse dormido is a very common expression meaning to fall asleep.

Literally, it is something like to remain/stay asleep, but in real usage it means to drift off or to nod off.

Why dormido?

Because dormido is functioning like an adjective here, describing the state the person ends up in.

Compare:

• se quedó dormido = he fell asleep
• se quedó dormida = she fell asleep
• se quedaron dormidos = they fell asleep

So dormido agrees with mi hermano, which is masculine singular.

Acabó is the pretérito indefinido form of acabar.

It is used because the sentence talks about a completed event in the past. The brother’s falling asleep is presented as a finished action in a story-like sequence.

• acabó = he ended up
• not acababa, which would sound more ongoing, habitual, or backgrounded

In a sentence like this, the preterite is the normal choice because the speaker is narrating what happened.

Because after a preposition like de, Spanish normally uses the infinitive, not a conjugated verb.

So:

• después de estudiar = after studying

This is a very common rule:

• antes de salir = before leaving
• después de comer = after eating
• sin decir nada = without saying anything

If you want a fully conjugated verb after después, you usually need después de que:

• después de que estudió = after he studied

But in your sentence, since the subject is understood to be the same person, después de estudiar is the most natural structure.

Because after verbs like estudiar, Spanish often uses school subjects without an article.

So:

• estudiar física
• estudiar matemáticas
• estudiar historia

This is very normal.

You may sometimes see the article in other contexts, for example when speaking more generally about the subject itself:

• La física es difícil = Physics is difficult

But after estudiar, leaving out the article is the standard pattern.

It means all afternoon or the whole afternoon.

This is a duration expression:

• toda la tarde = all afternoon
• toda la noche = all night
• todo el día = all day

In Spanish, these expressions often appear without a preposition when they express how long something lasted.

So:

• estudiar física toda la tarde = to study physics all afternoon

Because en el sofá is the normal Spanish way to say on the sofa when someone is sitting or lying on it.

Spanish often uses en where English uses on for furniture or locations of this kind:

• en la cama = in/on bed
• en el sofá = on the sofa
• en la silla can also be used depending on context

Sobre el sofá sounds more literal, like on top of the sofa, focusing on physical position rather than the normal place where someone sits or lies.

So here, en el sofá is the natural choice.

Yes. That is also correct.

Both of these are possible:

• Mi hermano acabó quedándose dormido...
• Mi hermano se acabó quedando dormido...

The meaning is basically the same. The difference is mainly where the reflexive pronoun se is placed.

Spanish allows this with infinitives and gerunds:

• attached to the non-finite form
• or placed before the conjugated verb

The version in your sentence sounds very natural and smooth.

Yes, they are important because they show the correct stress and sometimes distinguish forms.

• acabó stresses the last syllable: a-ca-BÓ
• física stresses the first syllable: FÍ-si-ca
• sofá stresses the last syllable: so-FÁ

In acabó, the accent also helps distinguish it from acabo:

• acabo = I end / I finish
• acabó = he/she ended

So the written accents matter both for pronunciation and for meaning.