Cuando llegamos a casa, mi hermana dejó la manta en el maletero y guardó el llavero en el recibidor.

Yes. For -ar verbs, the nosotros form in the present and the preterite is often identical:

• llegamos = we arrive
• llegamos = we arrived

You tell them apart from context. In this sentence, the other verbs are clearly in the past — dejó and guardó — so Cuando llegamos a casa is understood as When we got home.

The sentence presents the actions as completed events in a sequence:

• llegamos = we arrived / got home
• dejó = she left / put
• guardó = she put away / stored

Spanish uses the preterite for finished actions viewed as whole events.

If you used the imperfect, the meaning would change. For example:

• Cuando llegábamos a casa... = when we were arriving home / whenever we came home
• dejaba / guardaba = she used to leave / was leaving, used to put away / was putting away

So the preterite here fits a one-time past narrative.

In Spanish, home as a general destination is usually expressed without an article:

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

But if you mean a specific house, then you can use an article or another determiner:

• llegamos a la casa de Ana
• llegamos a mi casa

So a casa is the normal idiomatic way to say home.

Because Spanish is a pro-drop language: the verb endings usually show who the subject is.

• llegamos already tells you the subject is we
• dejó and guardó are third-person singular, and the noun mi hermana tells you who did those actions

So adding nosotros or ella would usually be unnecessary unless you want emphasis or contrast.

They are related, but not the same.

• dejar often means to leave, to put down, or to leave something somewhere
• guardar often means to put away, to store, to keep, or to put something in its proper place

So here:

• dejó la manta en el maletero suggests she left or placed the blanket in the boot/trunk
• guardó el llavero en el recibidor suggests she put away the keyring in the hallway/entry area

In other words, guardar often has more of a sense of tidying away or keeping something.

Because the preposition depends on the verb and the idea being expressed.

• llegar a
  • destination
    
    • llegamos a casa
• dejar en
  • location
    
    • dejó la manta en el maletero
• guardar en
  • location
    
    • guardó el llavero en el recibidor

So a marks the destination of arriving, while en marks where something is placed.

Because the sentence begins with a subordinate time clause:

• Cuando llegamos a casa, ...

In Spanish, when this kind of clause comes first, a comma is normally used before the main clause.

Compare:

• Cuando llegamos a casa, mi hermana dejó la manta...
• Mi hermana dejó la manta... cuando llegamos a casa.

In the second version, the time clause comes after the main clause, so there is normally no comma before cuando.

Yes, Spanish could omit it if the subject were already clear from context:

• Cuando llegamos a casa, dejó la manta en el maletero...

But that could be less clear, because dejó only tells us he/she/it. By adding mi hermana, the sentence explicitly identifies who did the actions.

So here mi hermana is useful for clarity.

Because it is the third-person singular preterite form of guardar.

For regular -ar verbs in the preterite:

• yo guardé
• tú guardaste
• él/ella guardó
• nosotros guardamos

That accent on -ó is part of the standard spelling. It also helps distinguish pronunciation and, in some cases, tense/person.

The same pattern appears in dejó from dejar.

The articles show you:

• la manta → feminine
• el maletero → masculine
• el llavero → masculine
• el recibidor → masculine

This matters because articles and other words must agree in gender and number.

A useful point for learners: while many nouns ending in -a are feminine and many ending in -o are masculine, you should always learn the noun together with its article, for example la manta, not just manta.

They are especially natural in Spain.

• maletero in Spain means the car boot/trunk
• recibidor means the entry hall, foyer, or hallway near the entrance

In other Spanish-speaking regions, different words may be more common for these ideas. For example, the word for trunk varies a lot across the Spanish-speaking world. But for Spain, maletero and recibidor are very normal choices.

Here it means when, referring to a specific past occasion.

That is because the sentence uses the preterite and describes a completed event in the past:

• Cuando llegamos a casa... = When we got home...

If the sentence were describing a repeated habit, cuando could mean whenever, and Spanish would often use the imperfect:

• Cuando llegábamos a casa, mi hermana dejaba...
• Whenever we got home / When we used to get home, my sister would leave...

So the tense helps determine whether cuando is about one event or repeated events.