La chica trabaja en la biblioteca.

La is the singular feminine definite article, meaning the.

Both chica and biblioteca are feminine singular nouns, so they use la:

• la chica
• la biblioteca

In Spanish, nouns have grammatical gender, so the article has to match the noun in gender and number.

Trabaja is the él/ella/usted form of the verb trabajar in the present tense.

Here is the pattern:

• yo trabajo = I work
• tú trabajas = you work
• él/ella/usted trabaja = he/she/you work

Since the subject is la chica, which is she, the correct form is trabaja.

Spanish often leaves out subject pronouns when they are not needed.

Because trabaja already tells you the subject is he/she/you (formal), and because la chica is already named, adding ella would usually be unnecessary.

So La chica trabaja en la biblioteca sounds natural.
You could say Ella trabaja en la biblioteca, but only if the context already makes it clear who ella is, or if you want emphasis.

Chica usually means girl, but depending on context it can also mean young woman.

In everyday Latin American Spanish, chica can refer to:

• a girl
• a young woman
• sometimes just a female person, informally

The exact age or nuance depends on context.

En commonly means in, at, or on, depending on context.

With places like la biblioteca, en is used to say where someone works, is located, or does something.

So in this sentence, en la biblioteca means the person works in / at the library.

Spanish uses en where English might choose either in or at.

No. Biblioteca means library, not bookstore.

This is a very common vocabulary trap for English speakers.

• biblioteca = library
• librería = bookstore

Even though library and librería look similar, librería does not mean library.

Yes, the most neutral word order is:

• La chica trabaja en la biblioteca.

Spanish word order is often flexible, but changes in order can sound more marked or emphasize different parts.

For example:

• En la biblioteca trabaja la chica.

This is possible, but it sounds more like you are emphasizing where she works.

For learners, the original order is the safest and most natural.

A simple pronunciation guide is:

• tra-BA-ha

The j in trabaja is pronounced like a strong, breathy h sound, not like the English j in job.

So it is tra-BA-ha, not tra-BA-ja in the English sense.

Also:

• the stress falls on ba
• the final a is clearly pronounced

The ending -a often suggests a noun is feminine, and that is true for both chica and biblioteca.

So here:

• chica → feminine
• biblioteca → feminine

But learners should remember that -a is only a common pattern, not a perfect rule. Some nouns ending in -a are masculine, such as el día or el problema.

That is why it is best to learn nouns together with their article:

• la chica
• la biblioteca

Yes. Muchacha is also a correct word and can mean girl or young woman.

However, chica is extremely common in everyday speech in much of Latin America. In many contexts, chica sounds more natural and more frequent.

So both are possible, but chica is often the more common everyday choice.