In bibliotheca tacere debes, quia alii libros legunt.

Because in takes the ablative when it means in or inside a place where something is happening.

So:

• in bibliotheca = in the library

If in showed motion into a place, Latin would usually use the accusative instead:

• in bibliothecam = into the library

Here there is no movement; it is just the location where you should be quiet.

Latin often expresses must do something with:

• debeo, debes, debet...
  • infinitive

So:

• debes = you must / you ought
• tacere = to be silent / to keep quiet

Together:

• tacere debes = you must be quiet or more literally you ought to be silent

This is a very common Latin pattern.

Debes is:

• 2nd person singular
• present tense
• active voice
• indicative mood

from the verb debeo, debere, meaning to owe, and often by extension to ought or must.

So debes means:

• you owe
• or, in this kind of sentence, you ought / you must

Because Latin usually does not need to state the subject pronoun if the verb ending already makes it clear.

Here, debes already means you must, so you is built into the verb ending.

Latin could add tu for emphasis:

• In bibliotheca tu tacere debes

But that would sound more emphatic, like you must be quiet.

It is an infinitive because it depends on debes.

After verbs like debeo in this meaning, Latin normally uses the infinitive to express the action that must be done:

• tacere debes = you must be quiet
• literally: you ought to be silent

The infinitive tacere comes from taceo, tacere, meaning to be silent or to keep quiet.

Latin word order is much more flexible than English word order because Latin uses case endings and verb endings to show grammatical relationships.

English depends heavily on position:

• Others read books is different from Books read others

Latin does not depend on word order in the same way because forms like alii, libros, and legunt already show who is doing what.

So In bibliotheca tacere debes, quia alii libros legunt is natural Latin, even though English would more naturally say:

• You must be quiet in the library, because other people are reading books

Latin often puts important setting information first, so In bibliotheca comes at the beginning.

Quia means because.

It introduces a clause giving the reason:

• quia alii libros legunt = because others are reading books

This is a straightforward causal clause. The verb in the clause, legunt, is in the indicative because it states a real reason or fact.

Alii is the subject of legunt, so it must be in the nominative plural.

It comes from alius, meaning other or another.

Here it is being used substantively, which means the adjective is functioning like a noun:

• alii = others or other people

So the idea is:

• alii legunt = others are reading

Latin often uses adjectives by themselves when the noun is understood.

So alii literally means something like:

• other ones
• other people

English does the same sometimes, as in the rich, the poor, or the brave.

In this sentence, the noun is not expressed, but the meaning is easy to understand from context: other people in the library.

Because libros is the direct object of legunt.

The verb legunt means they read, and the thing being read is books, so Latin puts books in the accusative case:

• nominative singular: liber = book
• accusative plural: libros = books as the object

So:

• alii libros legunt = others are reading books

Legunt is:

• 3rd person plural
• present tense
• active voice
• indicative mood

from lego, legere, meaning to read

So legunt means:

• they read
• they are reading

In this context, English often translates it naturally as are reading.

Because Latin does not have a separate tense that works exactly like the English progressive in every case.

The Latin present tense often covers both:

• they read
• they are reading

Context tells you which English translation sounds better. In this sentence, are reading is usually the most natural choice, because it explains why you should be quiet right now.

It can cover a range of meanings depending on context.

Debes can mean:

• you must
• you ought to
• you should

In this sentence, because it is about expected behavior in a library, you must be quiet is a very natural translation. But grammatically, the Latin itself is not always as harsh as an English command verb can sound.

Latin has no articles like English the and a/an.

So:

• bibliotheca can mean a library or the library
• libros can mean books or the books

The translator supplies the most natural English wording from context. Here in the library is the most natural English version.

Yes, in many cases it could.

For example, Latin could also say:

• Tacere debes in bibliotheca, quia alii libros legunt
• Quia alii libros legunt, in bibliotheca tacere debes

The basic meaning would stay the same, because the endings still show the grammar.

However, word order can change emphasis:

• In bibliotheca first emphasizes the setting
• tacere before debes puts focus on the action
• starting with quia would emphasize the reason first

So the meaning stays basically the same, but the emphasis can shift.