Mi sobrina guardó otra servilleta en el servilletero antes de sentarse a comer.

Guardó is the preterite form of guardar for él / ella / usted.

So:

• yo guardé
• tú guardaste
• él / ella guardó

The preterite is used for a completed action in the past. Here, the niece put away / placed / stored the napkin, and that action is viewed as finished.

So Mi sobrina guardó... means that she did that action at a specific moment before the next action.

Guardar often means to keep, put away, store, save, or put in its place, depending on context.

In this sentence, guardó otra servilleta en el servilletero does not mean saved in the sense of money or computer files. It means something like:

• put another napkin into the napkin holder
• placed another napkin in the holder
• put away another napkin in the napkin holder

So guardar is a very flexible verb, and context tells you which English translation works best.

In Spanish, otro / otra normally goes directly before the noun without un/una.

So you say:

• otra servilleta = another napkin
• otro libro = another book

Not:

• una otra servilleta ❌

This is different from English, where another historically includes the idea of an + other. In Spanish, otra already does that job by itself.

Servilletero means napkin holder or napkin dispenser, depending on the object.

It comes from:

• servilleta = napkin
• -ero = a common ending used for containers, places, or objects associated with something

So a servilletero is literally something related to napkins: a holder/container for them.

In Spain, this is a very normal everyday word.

Because the idea here is location inside / in the napkin holder, so Spanish uses en.

• en el servilletero = in the napkin holder

If you used a, it would suggest movement toward something, but with guardar, Spanish commonly uses en when you say where something ends up.

Compare:

• Guardó las llaves en el cajón. = She put the keys in the drawer.
• Metió las cartas en la caja. = He put the letters into the box.

So en fits the final location.

After antes de, Spanish uses an infinitive when the subject stays the same.

Here the subject is still mi sobrina, so Spanish says:

• antes de sentarse = before sitting down / before she sat down

Structure:

• antes de + infinitive

Examples:

• antes de comer = before eating
• antes de salir = before leaving
• antes de acostarse = before going to bed

Because sentarse is reflexive, the infinitive includes se.

Because sentarse means to sit down, while sentar usually means to seat someone/something or to sit something down.

So:

• sentarse = to sit down oneself
• sentar = to seat someone

Examples:

• Me senté. = I sat down.
• Sentó al niño en la silla. = He seated the child in the chair.

In your sentence, the niece is the one performing the action on herself, so Spanish uses the reflexive form sentarse.

Spanish often uses ponerse / sentarse / volver / salir / venir + a + infinitive to express the start or purpose of an action.

Here:

• sentarse a comer = to sit down to eat

This is a very common structure. It means that after sitting down, the person begins or is about to do the next action.

Other examples:

• sentarse a estudiar = to sit down to study
• sentarse a hablar = to sit down to talk

So a comer is not random: it tells you what she sat down to do.

Yes. The subject is still mi sobrina.

In Spanish, when you use antes de + infinitive, the subject is usually understood from the main clause if it is the same person.

So this sentence means:

• My niece put another napkin in the napkin holder before sitting down to eat.

Spanish does not need to repeat ella or mi sobrina here.

If the subject changed, Spanish would normally need a different structure, for example with a conjugated verb and a clearer subject.

Yes, antes de comer would be possible, but it would mean something slightly different or at least be less specific.

• antes de comer = before eating
• antes de sentarse a comer = before sitting down to eat

The original sentence gives a more precise sequence:

1. she put the napkin in the holder
2. then she sat down
3. then she ate

So sentarse a comer adds the image of taking a seat for the meal, not just eating in general.

Mi sobrina is the normal and neutral way to say my niece.

Spanish usually uses the possessive adjective before the noun:

• mi sobrina
• mi hermano
• mi casa

The form la sobrina mía is possible in some contexts, but it is usually more emphatic, contrastive, or stylistically marked. It is not the default choice here.

So for an ordinary sentence, Mi sobrina is exactly what you would expect.

Servilleta can refer to a napkin in general, whether paper or cloth, depending on context.

If needed, Spanish can be more specific:

• servilleta de papel = paper napkin
• servilleta de tela = cloth napkin

In your sentence, servilletero makes many people imagine paper napkins in a holder, but the exact type is not stated explicitly.

Spanish often leaves out subject pronouns because the verb form already tells you the person and number.

Here:

• guardó already tells you it is he/she/you formal
• and mi sobrina is explicitly stated anyway

So saying Ella guardó... would usually be unnecessary unless you wanted emphasis or contrast.

Spanish is a pro-drop language, which means subject pronouns are often omitted when they are clear from context.

Nothing in the grammar is uniquely Spain-only, but the sentence feels perfectly natural in Spain Spanish.

A few notes:

• servilleta and servilletero are standard in Spain.
• The overall word order is very natural and everyday.
• There is no specifically Latin American or specifically Peninsular slang here.

So a learner of Spanish from Spain can treat this as a completely normal standard sentence.