El socorrista vio que una niña se alejaba demasiado con el flotador y le pidió que volviera para seguir construyendo el castillo de arena con nosotras.

This is a very common Spanish tense contrast:

• vio is preterite: a completed event, seen as a whole.
• se alejaba is imperfect: an ongoing action or situation in progress.

So the sentence presents:

• El socorrista vio... = the lifeguard noticed/saw
• ...que una niña se alejaba demasiado... = that a girl was drifting/moving away too far

A natural way to understand it is:

• the lifeguard’s act of seeing happened at one moment
• the girl’s moving away was already in progress

This is one of the most typical uses of preterite + imperfect in narration.

Because the verb here is alejarse, which means to move away / to drift away / to go farther away.

• alejar usually means to move something away or to distance something
• alejarse means to move away oneself

So:

• La niña se alejaba = the girl was moving away
• without se, the meaning would change and sound incomplete or unnatural here

In this sentence, se is not really translated as a separate English word. It is just part of the verb alejarse.

Here demasiado means too much / too far.

It modifies the idea of moving away:

• se alejaba demasiado = she was going too far away / drifting too far

Even though demasiado often literally means too much, in English the most natural translation here is too far, because the verb is about distance.

Flotador in Spain usually refers to an inflatable floating aid, often a swim ring or armband-type float, depending on context. Here it most likely means with her float / with the inflatable ring.

So:

• una niña se alejaba demasiado con el flotador = a girl was drifting too far away with her float

The idea is that she was out in the water using a floating device.

Also note the article:

• el flotador = the float

Spanish often uses the definite article where English might use a possessive, especially when the context makes ownership obvious.

So English might say:

• with her float
• but Spanish says con el flotador

Le is the indirect object pronoun, meaning to her here.

So:

• le pidió que volviera = he asked her to come back

The full version could be:

• El socorrista le pidió a la niña que volviera

But Spanish often uses both the noun and the pronoun together, or just the pronoun if the person is already clear from context.

Here the girl is already mentioned, so le is enough.

Because the sentence means he asked her to come back, where one person asks another person to do something.

In Spanish, verbs like pedir, decir, querer, recomendar, insistir often use this pattern:

• verb + que + subjunctive

So:

• le pidió que volviera = he asked her to come back

If you say pidió volver, that would mean:

• he asked to come back or he requested to return
• in other words, the subject of both verbs would be the same person

But here the lifeguard is asking the girl to come back, so Spanish uses:

• pidió que + subjunctive

Because pedir que triggers the subjunctive.

Spanish uses the subjunctive after many verbs of:

• request
• influence
• command
• recommendation
• desire

Since pedir expresses a request, the following verb goes in the subjunctive:

• le pidió que volviera

This does not automatically mean uncertainty in the everyday English sense. It is just the normal grammar after a request.

Compare:

• Dice que vuelve = He says she is coming back
• Pide que vuelva = He asks that she come back

Because the main verb is in the past:

• pidió = he asked

When the main verb is in a past tense, Spanish normally uses the imperfect subjunctive in the subordinate clause:

• le pidió que volviera

This is a standard sequence:

• present main verb → present subjunctive
  
  • le pide que vuelva
• past main verb → imperfect subjunctive
  
  • le pidió que volviera

So volviera matches the past-time frame created by pidió.

Yes. Volviera and volviese are both valid forms of the imperfect subjunctive.

So both are grammatical:

• le pidió que volviera
• le pidió que volviese

In modern everyday Spanish, -ra forms like volviera are generally more common. The -se forms can sound a bit more formal, literary, or regional depending on the speaker, but they are not wrong at all.

Because para + infinitive is normally used when the subject is the same.

Here the sentence means the girl was asked to come back in order to continue building the sandcastle. The understood subject of seguir construyendo is the same girl who is supposed to volver.

So:

• le pidió que volviera para seguir construyendo...
• = he asked her to come back in order to continue building...

If Spanish wanted to make the next action belong clearly to a different subject, it would usually use:

• para que + subjunctive

For example:

• le pidió que volviera para que todas siguieran construyendo
• he asked her to come back so that they could all keep building

In your sentence, para seguir construyendo is the most natural choice.

Seguir + gerund means to continue doing something.

So:

• seguir construyendo = to continue building

This implies that the action had already started earlier. They were already making the sandcastle, and the girl had moved away, so the lifeguard asked her to come back to carry on with that activity.

If the sentence said just construir, it would sound more like to build, without emphasizing that the activity was already in progress.

Because castillo de arena is the standard Spanish expression for sandcastle.

Literally:

• castillo = castle
• de arena = of sand

Spanish often uses de + noun where English uses a compound noun:

• castillo de arena = sandcastle
• gafas de sol = sunglasses
• cepillo de dientes = toothbrush

Also, el castillo de arena uses the definite article because it refers to the specific sandcastle they were already building.

After a preposition like con, Spanish uses the prepositional pronoun:

• conmigo
• contigo
• con él / con ella
• con nosotros / con nosotras

So con nosotras means with us, specifically with us females.

That is why it is not:

• con nos ❌

Spanish does not use nos after a preposition.

The choice between nosotros and nosotras depends on the group:

• nosotros = us, masculine or mixed group
• nosotras = us, all female group

Since the sentence says con nosotras, the speaker is part of an all-female group.

The noun socorrista can be masculine or feminine depending on the article or other agreement.

So:

• el socorrista = the male lifeguard
• la socorrista = the female lifeguard

This is common with many nouns ending in -ista:

• el turista / la turista
• el dentista / la dentista
• el periodista / la periodista

In your sentence, el socorrista tells you the lifeguard is male.

The two ques do different but related jobs: they introduce subordinate clauses.

1. vio que...

   • El socorrista vio que una niña se alejaba...
   • Here que means that
   • = the lifeguard saw that a girl was drifting too far away

2. pidió que...

   • le pidió que volviera
   • Here que introduces the clause after a verb of request
   • = he asked her to come back

In English, the second one is often translated with an infinitive:

• asked her to come back

But Spanish normally uses:

• pedir que + subjunctive

So although both are que, the structures around them are different.

Because this is the first time the girl is being introduced into the story.

• una niña = a girl
• la niña = the girl

Spanish, like English, often introduces a new person or thing with the indefinite article and then may refer to them later with the definite article.

So this sentence treats her as a girl not previously identified:

• vio que una niña... = saw that a girl...

If she had already been mentioned earlier, la niña would be more likely.