En la biblioteca hay un estudiante muy callado que siempre se sienta al fondo.

Hay means “there is / there are” and is used to introduce the existence of something, usually with an indefinite article (un, una) or no article.

• Hay un estudiante = There is a student (we’re just saying that such a student exists there).
• Está el estudiante would suggest a specific, already known student: The student is there.
• Es is used for permanent characteristics, definitions, professions, etc., not for location like this.

So hay is correct because we’re introducing “a student”, not talking about a specific, previously known one.

Both are grammatically correct:

• En la biblioteca hay un estudiante muy callado…
• Hay un estudiante muy callado en la biblioteca…

Starting with “En la biblioteca” emphasizes the place first (Spanish often fronts the place or time):

• Focus on place: In the library, there is a very quiet student…
• More neutral: There is a very quiet student in the library…

It’s a stylistic choice, not a rule.

Because biblioteca is a feminine noun in Spanish. Feminine singular nouns usually take the article la:

• la biblioteca – the library
• una biblioteca – a library

Only masculine singular nouns take el (e.g. el libro, el coche). So el biblioteca would be incorrect.

Estudiante is a noun that can be masculine or feminine depending on the person:

• un estudiante – a (male) student
• una estudiante – a (female) student

The sentence uses un, so we are talking about a male student. If it were a female student, you’d say:

En la biblioteca hay una estudiante muy callada…

In Spanish, adjectives normally go after the noun:

• un estudiante callado – a quiet student
• un libro interesante – an interesting book

Putting it before (un callado estudiante) is possible in some cases but sounds literary, marked, or can even change the nuance. The natural, everyday order is:

un estudiante muy callado

Adjectives must agree in gender and number with the noun:

• un estudiante callado – masculine singular
• una estudiante callada – feminine singular

Because the article is un (masculine), estudiante is masculine in this sentence, so we use callado. If it were una estudiante, we’d say callada.

• callado = quiet
• muy callado = very quiet

Muy is an adverb that intensifies adjectives and other adverbs:

• muy alto – very tall
• muy rápido – very fast

Without muy, the student is quiet. With muy, they’re especially or noticeably quiet.

In this sentence, que is a relative pronoun meaning “who / that”:

un estudiante … que siempre se sienta al fondo
a student *who always sits at the back*

For people, Spanish can use que or quien/quienes. Here que is the most common, natural choice in everyday speech.

There are two different verbs:

• sentar(se) – to sit (down), to seat oneself
  
  • se sienta – he/she sits (down)
• sentir(se) – to feel
  
  • se siente – he/she feels

In this sentence we’re talking about sitting, so we must use sentar(se):

se sienta = he/she sits (down)
Not se siente, which would mean he/she feels.

Sentar on its own means “to seat someone else”:

• Sienta al niño en la silla. – He seats the child on the chair.

When you sit yourself down, Spanish usually uses the reflexive form sentarse:

• Se sienta – he/she sits (down) (literally: seats himself/herself)

So siempre se sienta = he always sits (down).
Siempre sienta would mean he always seats (someone else), which is not the intended meaning.

Spanish uses the simple present for habits and routines, just like English:

• Siempre se sienta al fondo. – He always sits at the back.
• Siempre come a las dos. – He always eats at two.

You don’t need an extra word like “usually”; siempre already expresses the repetition.

Al fondo literally means “to the back / at the back (of a room/place)”. It’s used for the area furthest from the entrance.

Al is the contraction of a + el:

• a + el fondo → al fondo

Spanish always contracts a el into al:

• Voy a el parque ✗
• Voy al parque ✓

Yes, a subtle one:

• al fondo – usually suggests movement or habitual position towards/at the back:
  
  • Se sienta al fondo. – He sits (goes to sit) at the back.
• en el fondo – emphasizes being in the back area (location), often a bit more static:
  
  • Está en el fondo. – He is at the back.

In many everyday contexts they overlap, but in your sentence al fondo with se sienta is the natural pair.

Spanish is a “pro-drop” language: it usually omits subject pronouns when the verb ending already shows who the subject is.

• se sienta → the ending -a (in sienta) and the context tell us it’s he/she.
• Adding Él se sienta al fondo is possible, but would add emphasis like “HE sits at the back (as opposed to others)”.

Here, the subject is clear from “un estudiante”, so no pronoun is needed.