Hoy hay menos gente en la biblioteca.

Because hay is the form used to say that something exists or there is/there are somewhere.

• Hay gente en la biblioteca = there are people in the library.
• Está/están are used more for saying where a specific person or thing is:
  • La profesora está en la biblioteca = the teacher is in the library.
  • Los estudiantes están en la biblioteca = the students are in the library.

So in this sentence, the idea is not about specific people already identified, but about the existence/quantity of people there, so hay is the right choice.

Because hay is an impersonal form of haber. In this use, it does not change for singular and plural.

So Spanish says:

• Hay un libro = there is a book
• Hay dos libros = there are two books
• Hay menos gente = there are fewer people

This is very different from English, where there is and there are must agree with the noun.

In English, there in there is / there are is a grammatical placeholder. Spanish does not need that placeholder.

So:

• There are people in the library → Hay gente en la biblioteca
• not Allí hay... unless you specifically want to stress there/over there

If you say allí or ahí, you are adding a real location word, not just the English dummy there.

Both are possible, but gente is very common and natural when talking about people in a place in a general way.

• menos gente = fewer people / less people in everyday speech
• menos personas = also correct, but a bit more formal or specific

In normal conversation, a Spanish speaker is very likely to say Hay menos gente.

Because gente is a collective singular noun. Grammatically, it is singular even though it refers to a group.

That means:

• Hay mucha gente = correct
• Hay muchas gentes = usually not correct in this meaning

And if you use an adjective with gente, it normally agrees in the singular:

• La gente está cansada = the people are tired

So gente behaves like a singular noun grammatically, even though its meaning is plural.

Here menos directly modifies the noun gente.

• menos gente = fewer people

You use menos de before a number or quantity expression:

• menos de veinte personas = fewer than twenty people
• menos de una hora = less than an hour

So:

• Hay menos gente = correct
• Hay menos de gente = incorrect

By itself, it just means fewer people, compared with some context that is understood.

That comparison could be:

• than usual
• than yesterday
• than last week
• than in another place

Spanish, like English, often leaves that comparison unstated if it is obvious from the situation.

If you want to make it explicit, you can say:

• Hoy hay menos gente que ayer en la biblioteca = Today there are fewer people in the library than yesterday.
• Hoy hay menos gente de lo normal en la biblioteca = Today there are fewer people than usual in the library.

Because en is the normal preposition for being in a place.

• en la biblioteca = in the library

Compare:

• Voy a la biblioteca = I am going to the library
• Estoy en la biblioteca = I am in the library
• Hay menos gente en la biblioteca = There are fewer people in the library

So a is for movement toward a place, while en is for location.

Yes. Spanish word order is more flexible than English.

These are all possible:

• Hoy hay menos gente en la biblioteca.
• En la biblioteca hay menos gente hoy.
• Hoy en la biblioteca hay menos gente.

The original version is very natural. Starting with Hoy sets the time first, which is common.

Changing the order usually changes emphasis, not the basic meaning.

No. Hoy can go in different places depending on emphasis.

For example:

• Hoy hay menos gente en la biblioteca = neutral, very natural
• Hay menos gente en la biblioteca hoy = also correct, with today sounding a bit more added or contrasted
• En la biblioteca hoy hay menos gente = more emphasis on the location first

Putting hoy first is probably the most straightforward choice.

Because gente here is being used in a general, indefinite way.

• Hay gente = there are people
• Hay menos gente = there are fewer people

If you say la gente, you usually mean the people in a more definite or generic sense:

• La gente está preocupada = people / people in general are worried
• La gente de la biblioteca = the people in the library

After hay, Spanish often uses a noun without the article when introducing or counting something in an indefinite way.

Yes, it can be. Biblioteca means library, not bookshop.

A common mistake is to confuse:

• biblioteca = library
• librería = bookshop / bookstore

So in this sentence, la biblioteca definitely means the library.

In standard Spanish from Spain, hay is pronounced like eye in English, roughly /ai/.

Also remember:

• the h is silent
• hoy also has a silent h, and sounds roughly like /oi/

So:

• hoy ≈ oy
• hay ≈ eye

Yes, but it is a different structure and slightly different in focus.

• Hoy hay menos gente en la biblioteca focuses on the number of people.
• La biblioteca está menos llena hoy focuses on how full the library is.

Both are natural, but the original sentence is the more direct way to talk about there being fewer people.