Cuando llegué a casa, mi hermana ya había dejado el llavero en el recibidor.

Llegué is the preterite of llegar in the yo form: I arrived.

It is used because the sentence refers to a completed action in the past: the speaker arrived home at a specific moment. In Spanish, the preterite is very common for that kind of finished event.

So:

• cuando llegué = when I arrived
• not a repeated or ongoing action, but one completed event

Había dejado is the pluperfect (also called the past perfect): had left / had put down.

It is used because the sister’s action happened before another past action.

Timeline:

• First: mi hermana había dejado el llavero...
• Later: yo llegué a casa

So Spanish uses the pluperfect to show an earlier past relative to another past event.

Compare:

• Cuando llegué a casa, mi hermana ya había dejado el llavero...
  = When I arrived home, my sister had already left the keyring...

• Cuando llegué a casa, mi hermana dejó el llavero...
  = When I arrived home, my sister left / put down the keyring...
  This sounds like her action happened around that moment, not earlier.

Ya here means already.

It emphasizes that the action was completed before the speaker arrived:

• ya había dejado = had already left / had already put

Without ya, the sentence would still be correct, but ya makes the sequence clearer and more natural.

Compare:

• había dejado = had left
• ya había dejado = had already left

In Spanish, casa often appears without an article in expressions involving going home, coming home, returning home, especially when it means one’s home rather than a particular house as a building.

So:

• llegar a casa = to arrive home
• ir a casa = to go home
• volver a casa = to return home

This is a very common fixed pattern.

But if you mean a specific house as a physical place, you may see the article:

• llegué a la casa de Ana = I arrived at Ana’s house

So in your sentence, a casa is the natural way to say home.

This is a very common question.

• a casa shows movement toward home
  
  • Llegué a casa = I arrived home
• en casa shows location at home
  
  • Estoy en casa = I am at home

So:

• a = destination
• en = location

In your sentence, the speaker is talking about arriving, so a casa is needed.

Because cuando is being used as a conjunction, meaning when, not as a question word.

So:

• Cuando llegué a casa... = When I arrived home...
  No accent, because it is connecting parts of the sentence.

But in direct or indirect questions, it does take an accent:

• ¿Cuándo llegaste? = When did you arrive?
• No sé cuándo llegó. = I don’t know when he arrived.

Rule of thumb:

• cuando = no accent when it connects
• cuándo = accent when it asks or implies a question

This happens because of Spanish spelling rules.

The verb is llegar. In the yo preterite, regular -ar verbs normally end in -é:

• hablé
• trabajé
• llegué

But if Spanish wrote llegé, the g before e would sound like the strong Spanish j sound. To keep the original hard g sound from llegar, Spanish inserts u:

• llegué = hard g sound preserved

The accent on -é is there because that is the normal yo preterite ending for -ar verbs.

So llegué reflects both:

1. the normal preterite ending -é
2. a spelling change to preserve pronunciation

Spanish often uses the definite article (el, la, los, las) where English would use a possessive like his, her, my—especially when ownership is already clear from the context.

Here, we already know we are talking about my sister, so el llavero sounds natural.

So:

• mi hermana dejó el llavero
  = my sister left the keyring

This does not mean it cannot be hers. It just means Spanish does not always repeat the possessive if it is obvious.

If you said su llavero, it would put more focus on whose keyring it was:

• mi hermana había dejado su llavero...
  = my sister had left her keyring...

That is also possible, but not necessary.

In this sentence, dejar means something like to leave, to put down, or to leave behind, depending on how you interpret the context.

So:

• había dejado el llavero en el recibidor
  can mean
  had left the keyring in the hall
  or
  had put the keyring down in the hall

Dejar is a very flexible verb in Spanish. Common meanings include:

• leave something somewhere
• put something somewhere and leave it there
• allow / let in other contexts

Here it is about placing and leaving the keyring in that spot.

In Spain, recibidor usually means the entrance hall, entryway, or small hall just inside the front door.

It is a very natural word in Peninsular Spanish.

Depending on the house, it might be translated as:

• hall
• entry hall
• entrance area
• foyer sometimes, though that can sound more formal in English

In other Spanish-speaking regions, people might more often say things like:

• la entrada
• el vestíbulo
  depending on the context.

Yes. Spanish word order is fairly flexible, as long as the meaning stays clear.

Your sentence is very natural:

• Cuando llegué a casa, mi hermana ya había dejado el llavero en el recibidor.

But you could also say:

• Mi hermana ya había dejado el llavero en el recibidor cuando llegué a casa.

That means essentially the same thing.

The version with Cuando llegué a casa at the beginning is often used to set the scene first. It sounds very natural in storytelling.

Yes, it is normal and recommended here.

When a subordinate time clause like Cuando llegué a casa comes before the main clause, Spanish usually writes a comma after it:

• Cuando llegué a casa, mi hermana ya había dejado...

If the order is reversed, the comma is often omitted:

• Mi hermana ya había dejado el llavero en el recibidor cuando llegué a casa.

So the comma is not random; it reflects the sentence structure.

You could, but the meaning would change.

• Cuando llegué a casa = When I arrived home
  A specific completed moment

• Cuando llegaba a casa = When I was arriving home / When I used to arrive home
  This sounds more ongoing, descriptive, or habitual depending on context

In your sentence, the speaker is referring to one completed arrival, so llegué is the best choice.

Yes. A very natural alternative is:

• Al llegar a casa, mi hermana ya había dejado el llavero en el recibidor.

Al + infinitive often means upon / when doing something.

So:

• Al llegar a casa = On arriving home / When I arrived home

This structure is common in Spanish and sounds natural, but your original sentence with Cuando llegué a casa is just as correct.

Spanish often drops subject pronouns like yo, tú, él, because the verb ending already shows the person. But mi hermana is not a pronoun; it is a full noun phrase, and here it is useful because it tells us who had left the keyring.

You could not simply omit it unless the context had already made the subject completely clear.

For example:

• Cuando llegué a casa, ya había dejado el llavero en el recibidor.
  This would normally mean I had already left the keyring, because había dejado without another subject tends to refer back to the speaker.

So mi hermana is necessary to identify the sister as the one who performed that action.