¿Dónde está Laura? Estará en la biblioteca.

In Spanish, the simple future is often used to make a guess or assumption about the present.

So:

• ¿Dónde está Laura?
• Estará en la biblioteca.

does not really mean She will be in the library in a future-time sense. It means something more like:

• She’s probably in the library
• She’ll be in the library, I suppose

This is a very common use of the future in Spanish.

Estará is the 3rd person singular future form of estar.

The future tense is made by adding endings to the full infinitive:

• estaré = I will be
• estarás = you will be
• estará = he/she/it will be; you (formal) will be
• estaremos = we will be
• estaréis = you all will be
• estarán = they/you all will be

Here, estará refers to Laura, so it means she will be or, in this context, she is probably.

Spanish uses estar for location.

So:

• Laura está en la biblioteca = Laura is in the library

You would not use ser for someone’s physical location in a sentence like this.

A quick rule:

• ser = identity, origin, characteristics, etc.
• estar = location, condition, state

Since the sentence is about where Laura is, estar is the correct verb.

Spanish often leaves out subject pronouns when they are clear from the context or the verb ending.

Here, the first sentence mentions Laura, so the second sentence does not need ella:

• ¿Dónde está Laura? Estará en la biblioteca.

Adding ella is possible, but usually unnecessary:

• Ella estará en la biblioteca.

Spanish prefers to omit pronouns unless you want emphasis, contrast, or clarity.

In Spanish, word order in questions is not the same as in English.

The natural order here is:

• ¿Dónde está Laura?

Spanish does not use do/does like English, and it does not need the same kind of inversion pattern English uses.

With question words such as dónde, qué, cuándo, etc., the verb often comes early:

• ¿Dónde está Laura?
• ¿Qué hace Marta?
• ¿Cuándo llega el tren?

¿Dónde Laura está? sounds unnatural in standard Spanish.

Because en is used for being in a place, while a is used for movement toward a place.

• Está en la biblioteca = She is in the library
• Va a la biblioteca = She is going to the library

So in this sentence, Laura is being located inside / at the library, not moving toward it.

Spanish uses the definite article more often than English does.

So en la biblioteca is the normal way to say in the library / at the library.

English often drops the article in some expressions, but Spanish usually keeps it:

• en la biblioteca
• en el banco
• en la oficina
• en el supermercado

Saying just en biblioteca would usually sound incomplete or unnatural in standard Spanish.

Dónde has a written accent because it is an interrogative word here.

Compare:

• ¿Dónde está Laura? = Where is Laura?
• No sé dónde está Laura. = I don’t know where Laura is.

In both cases, dónde keeps the accent because it introduces a question, direct or indirect.

Without the accent, forms like donde are used in other structures, usually meaning where in a relative sense:

• La biblioteca donde estudia Laura = the library where Laura studies

The accent marks show the correct stress pattern.

• está is stressed on the last syllable
• estará is also stressed on the last syllable

This matters because Spanish pronunciation is strongly linked to written accents.

Also, está with an accent is the verb form:

• Laura está aquí

This is different from esta without an accent, which is a demonstrative determiner or pronoun:

• esta biblioteca = this library

So the accent helps both with pronunciation and with distinguishing forms.

Yes. That would be the normal choice if you are stating it as a fact.

• Laura está en la biblioteca. = Laura is in the library.
• Laura estará en la biblioteca. = Laura is probably in the library / I suppose she’s in the library.

So the difference is mainly:

• está = certainty, direct statement
• estará = guess, assumption, probability

Yes, very common.

In Spain, and also in much of the Spanish-speaking world, the future tense is often used for supposition or probability:

• Serán las cinco. = It’s probably five o’clock.
• Estará en casa. = He/She is probably at home.
• Tendrán hambre. = They’re probably hungry.

So Estará en la biblioteca is a very natural sentence in Spanish from Spain.

Yes. A very common alternative is deber de + infinitive:

• Debe de estar en la biblioteca.

This also means:

• She must be / She’s probably in the library

Both are natural, but the nuance can vary a little depending on speaker and context:

• Estará en la biblioteca = probably / I guess she’s in the library
• Debe de estar en la biblioteca = she must be / she’s probably there

In everyday Spanish, both patterns are useful to know.