La niña quiere vestirse ella sola.

Spanish often uses the definite article more than English does. La niña means the girl.

If you said just niña, it would usually sound incomplete here, unless it appeared in a special context such as a label, headline, or someone calling out to a girl.

Quiere is the third-person singular form of querer in the present tense.

That matches la niña, which is she:

• yo quiero = I want
• tú quieres = you want
• él/ella quiere = he/she wants

So La niña quiere... means The girl wants...

After querer, Spanish normally uses another verb in the infinitive, just like English uses to + verb:

• quiere comer = wants to eat
• quiere dormir = wants to sleep
• quiere vestirse = wants to get dressed

So quiere vestirse is the standard structure: wants to get dressed.

Because here the action is reflexive: the girl is dressing herself, not dressing someone else.

• vestir = to dress someone / to clothe
• vestirse = to get dressed / to dress oneself

Compare:

• La madre quiere vestir a la niña. = The mother wants to dress the girl.
• La niña quiere vestirse. = The girl wants to get dressed.

In Spanish, when a reflexive verb is in the infinitive, the reflexive pronoun can be attached to the end:

• vestirse
• lavarse
• peinarse

That is completely normal after another verb like querer.

You could also say:

• La niña se quiere vestir ella sola.

This is also possible, but quiere vestirse is often the cleaner and more common pattern for learners to notice first.

Ella sola adds the idea of by herself / on her own.

So:

• La niña quiere vestirse. = The girl wants to get dressed.
• La niña quiere vestirse ella sola. = The girl wants to get dressed by herself.

It emphasizes independence: she does not want help.

The pronoun ella is not required for basic grammar here. It is used for emphasis.

Spanish often adds a pronoun like yo, tú, él, ella when the speaker wants to stress it:

• Yo lo hago. = I do it. / I do it.
• Ella sola = she herself / all by herself

So ella strengthens the idea that the girl wants to do it personally, without help.

Because sola agrees with la niña / ella, which is feminine singular.

Agreement in Spanish means adjectives change form to match the noun or pronoun:

• niño solo = boy alone
• niña sola = girl alone
• niños solos = boys alone
• niñas solas = girls alone

Here the person is a girl, so sola is the correct form.

Here it means by herself / on her own.

In many contexts, solo/sola can mean either:

• physically alone
• doing something without help

In this sentence, because it follows vestirse, the natural meaning is without help.

Yes. La niña quiere vestirse sola is grammatical and very natural.

Adding ella makes the independence more emphatic:

• vestirse sola = get dressed on her own
• vestirse ella sola = get dressed all by herself

So the version with ella sounds a bit stronger or more insistent.

Not always, but the end position is very natural here.

You may also hear variations like:

• La niña ella sola quiere vestirse.
• La niña quiere ella sola vestirse.

But these are less neutral and can sound more marked or stylistically different. The most straightforward version is:

• La niña quiere vestirse ella sola.

The ñ is a separate letter in Spanish. It is pronounced like the ny sound in canyon.

So niña sounds roughly like NEE-nya.

That makes it different from nina, which would have a normal n sound and is a different word spelling.