La niña se aleja de la orilla y la madre se pone nerviosa.

Because alejarse is the usual verb for to move away / go away / get farther away from something.

There is an important difference:

• alejar = to move something or someone away
• alejarse = to move away oneself

For example:

• La madre aleja a la niña de la orilla. = The mother moves the girl away from the shore.
• La niña se aleja de la orilla. = The girl moves away from the shore.

So here, the girl herself is the one moving away, which is why Spanish uses se aleja.

Because alejarse normally goes with de to show what someone is moving away from.

So:

• alejarse de algo/alguien = to move away from something/someone

Examples:

• Se aleja de la puerta. = She moves away from the door.
• Se aleja del grupo. = He moves away from the group.

Using de here is just the normal pattern of the verb.

Orilla means the edge or bank/shore of something, especially water.

Depending on context, it could be:

• the shore
• the riverbank
• the water’s edge

So in this sentence, it probably means the edge of the sea, a river, a lake, or similar.

Spanish often repeats the subject when a new person becomes the subject of the next verb.

In the first part, the subject is la niña.
In the second part, the subject changes to la madre.

If you said only y se pone nerviosa, it could sound as if the girl is the one getting nervous. By repeating la madre, Spanish makes the change of subject clear.

Here, ponerse + adjective is a very common structure meaning to become / to get a certain way.

So:

• ponerse nervioso/a = to become nervous
• ponerse triste = to become sad
• ponerse contento/a = to become happy

So se pone nerviosa does not literally mean puts herself nervous. It is just the normal Spanish way to say gets nervous or becomes nervous.

The difference is mainly this:

• se pone nerviosa = she becomes / gets nervous
• está nerviosa = she is nervous

In this sentence, the mother reacts to what is happening, so se pone nerviosa emphasizes the change of state: she was fine, and then she gets nervous.

If you used está nerviosa, it would describe her condition, but not focus as strongly on the moment of change.

Because adjectives in Spanish usually agree with the noun they describe.

Here, nerviosa describes la madre, which is feminine singular.

So:

• el padre está nervioso
• la madre está nerviosa
• los niños están nerviosos
• las niñas están nerviosas

The ending changes to match gender and number.

Both verbs are in the present indicative:

• se aleja
• se pone

They are both third person singular.

In English, this kind of Spanish present can often be translated in different ways depending on context:

• The girl moves away from the shore and the mother gets nervous.
• The girl is moving away from the shore and the mother is getting nervous.

If this is describing a scene right in front of you, the English progressive form (is moving, is getting) often sounds more natural.

Literally, it means the mother.

But in context, most people would naturally understand it as the girl’s mother.

Spanish does not always need to make that relationship explicit if the situation already makes it clear. If the speaker wanted to be more explicit, they could say su madre.

So:

• la madre = the mother
• su madre = her mother / his mother / their mother

In this sentence, la madre is enough because the scene already suggests who she is.

Niña usually refers to a child or young girl.

Chica can mean girl, but it often sounds older, like a girl / young woman rather than specifically a small child.

So niña is the better choice if the person is clearly a child.

A simple guide:

• niña: the ñ sounds roughly like ny in canyon
• orilla: for most speakers in Spain, ll sounds like the y in yes

Approximate pronunciations:

• niña ≈ NEEN-ya
• orilla ≈ oh-REE-ya
• madre ≈ MA-dreh

Also, the stress is on:

• NIña
• oRIlla
• MAdre

The ñ is especially important, because n and ñ are different letters in Spanish.