Cuando llegué a casa, mi padre ya estaba dormido en el sofá.

Because the sentence is contrasting two different kinds of past actions:

• llegué = preterite, used for a completed action: I arrived
• estaba dormido = imperfect, used for a background state: he was asleep

So the idea is:

• Cuando llegué a casa... = the moment I arrived home
• ...mi padre ya estaba dormido... = my father was already in the state of being asleep

This is a very common Spanish pattern: a completed event interrupts or happens during an ongoing situation.

Both can be possible in some contexts, but they do not feel exactly the same.

• estaba dormido = was asleep
  This describes a state
• dormía = was sleeping / used to sleep
  This focuses more on the action of sleeping

In this sentence, Spanish naturally prefers estaba dormido because the important idea is that the father was already asleep when the speaker arrived.

So:

• Mi padre estaba dormido = my father was asleep
• Mi padre dormía = my father was sleeping

The first one matches the English meaning was asleep more closely.

Dormido is the past participle of dormir, but in this sentence it is functioning like an adjective.

• estar dormido = to be asleep

So dormido here describes the father’s condition, not a completed action.

Compare:

• ha dormido = he has slept → participle used in a verb tense
• está dormido = he is asleep → participle used adjectivally

In your sentence, it is the second use.

In Spanish, ir/llegar/volver a casa is a fixed, very common expression meaning to go/arrive/return home.

So:

• llegué a casa = I arrived home

Spanish usually does not use the article here.

But if you are talking about a specific house rather than the idea of home, then you can use the article:

• llegué a la casa de mi amigo = I arrived at my friend’s house

So a casa means home, not just to the house.

Because Spanish often drops subject pronouns when the verb ending already makes the subject clear.

• llegué clearly means I arrived

So yo llegué is grammatically possible, but usually unnecessary unless you want emphasis or contrast.

For example:

• Cuando llegué a casa... = neutral
• Cuando yo llegué a casa... = more emphatic, maybe contrasting with someone else

This omission of subject pronouns is very normal in Spanish.

Ya here means already.

It shows that by the time the speaker arrived home, the father had fallen asleep before that moment.

• mi padre estaba dormido = my father was asleep
• mi padre ya estaba dormido = my father was already asleep

So ya helps express the timeline clearly.

Because this sentence refers to a real event in the past.

• Cuando llegué a casa... = when I arrived home

This is a specific event that actually happened, so Spanish uses the indicative.

You usually get the subjunctive with cuando when the event is in the future or not yet realized:

• Cuando llegue a casa, te llamaré = When I get home, I’ll call you

So:

• past, real event → cuando llegué
• future/unknown event → cuando llegue

Spanish often uses possessives like mi, tu, su with family members when needed for clarity.

• mi padre = my father

You can sometimes omit the possessive in certain contexts, especially with verbs like tener, or in more formal/literary styles, but here mi padre is the natural and normal choice.

It sounds clear and idiomatic.

En usually means in, on, or at, depending on context.

With furniture like sofá, en is the normal preposition:

• en el sofá = on the sofa / on the couch

Spanish uses en where English may choose either in or on. So you should not translate prepositions too literally word for word.

Because the stress falls on the last syllable: so-FÁ.

Spanish spelling rules say that words ending in a vowel, n, or s are normally stressed on the second-to-last syllable unless there is a written accent.

Without the accent, sofa would be pronounced roughly SO-fa, which is wrong in Spanish.

So the accent mark shows the correct stress:

• sofá

The accent in llegué shows the correct stress and form of the preterite:

• llegué = I arrived

Also, the spelling gué is used so that the g keeps its hard sound before e.

Compare:

• llegué
• llegamos
• llegaste

The accent helps mark the correct written form of the yo preterite ending -é.

It could, but it would mean something different.

• Cuando llegué a casa... = when I arrived home
  A single completed event
• Cuando llegaba a casa... = when I was arriving home / when I used to arrive home
  More ongoing, habitual, or incomplete

Your original sentence is about one specific moment, so llegué is the correct choice.

Yes, that is grammatical. Spanish allows some flexibility with adverb placement.

• mi padre ya estaba dormido
• mi padre estaba ya dormido

Both are possible, but ya estaba dormido is more natural and common here.

Putting ya before estaba sounds smoother and is what most learners should use by default.

Yes.

Since dormido is acting like an adjective here, it agrees with the noun it describes.

• mi padre estaba dormido → masculine singular
• mi madre estaba dormida → feminine singular
• mis padres estaban dormidos → masculine/mixed plural
• mis hermanas estaban dormidas → feminine plural

So the ending changes to match the person or people being described.

Sometimes, but they are not exactly the same.

• sofá = sofa / couch
• sillón = armchair / large comfortable chair

So if the father was on a couch, sofá is correct. If he was in a big armchair, sillón would be better.

In Spain, both words are common, but they refer to different pieces of furniture.

It is:

• Cuando llegué a casa = subordinate time clause
• mi padre ya estaba dormido en el sofá = main clause

So literally the structure is:

When I arrived home, my father was already asleep on the sofa.

Spanish often places the time clause first, just like English, but you could also reverse the order:

• Mi padre ya estaba dormido en el sofá cuando llegué a casa.

That means the same thing.