Antes de entrar a la biblioteca, apago el celular.

Antes de means before.

In Spanish, antes de + infinitive is a very common pattern for saying before doing something:

• Antes de salir = before leaving
• Antes de comer = before eating
• Antes de entrar = before entering

So Antes de entrar a la biblioteca literally means Before entering the library.

After antes de, Spanish usually uses an infinitive when the subject is the same as in the main clause.

Here, the same person does both actions:

• entrar = to enter
• apago = I turn off

So the sentence means:

• Before entering the library, I turn off my cellphone.

Because I am both the one entering and the one turning off the phone, Spanish uses antes de + infinitive.

If the subject changed, you would normally use antes de que + subjunctive instead:

• Antes de que él entre a la biblioteca, apago el celular.
• Before he enters the library, I turn off the cellphone.

In much of the Spanish-speaking world, especially in Latin America, entrar a is very common and means to enter / to go into.

So:

• entrar a la biblioteca = to enter the library
• entrar al edificio = to enter the building

You may also hear entrar en in some regions or styles:

• entrar en la biblioteca

Both are grammatical, but entrar a is extremely common in Latin American Spanish.

It can match either one, depending on context.

Spanish entrar a often covers both:

• to enter
• to go into

So Antes de entrar a la biblioteca could be understood as:

• Before entering the library
• Before going into the library

English chooses one or the other depending on style, but Spanish does not need that distinction here.

Apago is the first-person singular present form of apagar:

• yo apago = I turn off

In Spanish, the present tense is often used for habitual actions or general routines. So this sentence usually suggests something like:

• Before entering the library, I turn off my cellphone.
• In other words: this is something I normally do.

If you wanted to talk about one specific past event, you would use a past tense:

• Antes de entrar a la biblioteca, apagué el celular.
• Before entering the library, I turned off my cellphone.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• apago already tells you the subject is yo = I

So:

• Apago el celular = I turn off the cellphone

Adding yo is possible, but it usually adds emphasis, contrast, or clarity:

• Yo apago el celular, pero él no.
• I turn off my cellphone, but he doesn’t.

In a simple sentence like this, leaving out yo sounds natural.

In Spanish, it is very common to use the definite article (el, la, los, las) instead of a possessive like my when the owner is obvious from context.

So:

• apago el celular literally = I turn off the cellphone
• natural English meaning = I turn off my cellphone

Because the speaker is obviously talking about their own phone, Spanish does not need mi.

Both are possible:

• Apago el celular = natural, common
• Apago mi celular = also correct, slightly more explicit

Yes. Celular is a masculine noun in this meaning, so it takes el:

• el celular

This is simply the grammatical gender of the noun. It does not mean anything special beyond normal noun gender.

Also, in Latin America, celular is a very common word for cell phone / mobile phone.

In Spain, people more often say:

• el móvil

But in Latin America, el celular is the safer choice.

Spanish usually uses an article with nouns in cases where English may or may not use one.

So:

• la biblioteca = the library

Without the article, biblioteca by itself would usually not sound complete in this sentence.

Also, la biblioteca may refer to:

• the specific library relevant in context, or
• the place as an institution you are about to enter

Spanish often uses the definite article in everyday expressions where English might be less explicit.

The comma separates the introductory phrase from the main clause:

• Antes de entrar a la biblioteca, = introductory time expression
• apago el celular. = main clause

This is similar to English:

• Before entering the library, I turn off my cellphone.

The comma helps readability and is standard punctuation here.

That would change the grammar and possibly the meaning.

Antes de + infinitive is used when the subject is the same:

• Antes de entrar a la biblioteca, apago el celular.
• I enter, and I turn off the phone.

Antes de que + subjunctive is used when the subject is different, or when you want a full clause:

• Antes de que él entre a la biblioteca, apago el celular.
• Before he enters the library, I turn off the cellphone.

If you say Antes de que entre a la biblioteca without making the subject clear, it may sound ambiguous or less natural in this context. For the original sentence, antes de entrar is the normal choice.

Usually, it describes a habit or routine, because the main verb is in the present tense:

• Antes de entrar a la biblioteca, apago el celular.
• Before entering the library, I turn off my cellphone.

This sounds like something the speaker normally does whenever they go in.

However, depending on context, Spanish present tense can sometimes also describe an immediate or near-future action. Still, the most natural reading here is habitual.

Yes. Spanish allows some flexibility.

For example:

• Apago el celular antes de entrar a la biblioteca.

This means the same thing: I turn off my cellphone before entering the library.

The difference is mainly stylistic:

• Antes de entrar a la biblioteca, apago el celular.
  Focuses first on the time frame.
• Apago el celular antes de entrar a la biblioteca.
  Focuses first on the action.

Both are natural.