Mi hermana empezó a acomodar los cojines del sillón mientras yo recogía la mesa.

Because empezó a + infinitive means started to do something. It focuses on the beginning of the action.

• empezó a acomodar = she started arranging/straightening the cushions
• acomodó = she arranged the cushions

So the original sentence highlights the moment the action began, not just the action itself.

Because empezar is normally followed by a + infinitive when it means to start doing something.

This is a fixed pattern:

• empezar a hablar
• empezar a comer
• empezar a acomodar

So empezó a acomodar is the standard structure.

This is a very common Spanish past-tense contrast.

• empezó is preterite because it refers to a completed event: the moment her sister started
• recogía is imperfect because it describes an action already in progress in the background

So the idea is:

• one action began at a specific moment
• another action was already going on at that time

This is exactly the kind of contrast Spanish often makes with preterite + imperfect.

Mientras means while and connects two actions happening at the same time.

In this sentence, it links:

• Mi hermana empezó a acomodar los cojines del sillón
• yo recogía la mesa

So it shows simultaneity: one thing happened while another was going on.

Here, acomodar means something like to arrange, to put in place, or to straighten up.

A useful note for Spain: acomodar is understood, but in everyday Peninsular Spanish, people often say things like:

• colocar los cojines
• arreglar los cojines
• poner bien los cojines

So the sentence is understandable, but acomodar can sound a bit less typical for Spain than for some Latin American varieties.

In Spain, sillón usually means armchair or large comfy chair, not normally sofa.

That matters because English couch/sofa is usually sofá in Spain.

So los cojines del sillón would most naturally mean the cushions of the armchair. Depending on context, that may sound a little unusual if you imagine a single armchair with several cushions, but it is still perfectly possible.

Because del sillón means of the armchair / belonging to the armchair.

• los cojines del sillón = the armchair’s cushions
• los cojines en el sillón = the cushions on/in the armchair

So del identifies which cushions they are, while en would focus more on location.

In this context, recoger la mesa is an idiomatic expression meaning to clear the table after a meal.

It does not mean physically lifting the table.

This is very common in Spanish:

• poner la mesa = to set the table
• recoger la mesa = to clear the table

So this is a phrase you should learn as a chunk.

No, it is not strictly necessary. Spanish often drops subject pronouns because the verb ending already shows the subject.

So you could also say:

• Mi hermana empezó a acomodar los cojines del sillón mientras recogía la mesa.

Including yo adds emphasis or contrast, especially because the sentence mentions mi hermana first. It helps make the contrast clearer:

• my sister was doing one thing
• I was doing another

Because Spanish often uses the definite article when the reference is clear from context.

Here, la mesa means the table, but in natural English the idea is often the table we were using, especially the dining table.

Spanish does this a lot when possession is obvious, so it does not always need a possessive like English does.

Yes. Estaba recogiendo la mesa would also be correct.

The difference is mainly nuance:

• recogía la mesa = imperfect; simple narrative background
• estaba recogiendo la mesa = explicitly stresses that the action was in progress

Both can work here. The version with recogía is very natural in storytelling.

Yes. You could also say:

Mientras yo recogía la mesa, mi hermana empezó a acomodar los cojines del sillón.

The meaning stays basically the same. The difference is just emphasis:

• starting with Mi hermana... puts the focus on your sister’s action first
• starting with Mientras yo... sets the scene first

Both are natural.

Usually, no comma is needed in the original order:

Mi hermana empezó a acomodar los cojines del sillón mientras yo recogía la mesa.

That is the normal punctuation.

If the mientras clause comes first, a comma is normally used after it:

Mientras yo recogía la mesa, mi hermana empezó a acomodar los cojines del sillón.