Alii in bibliotheca libros veteres legunt.

Alii is the subject of the sentence.

It is the masculine nominative plural form of alius, meaning other. Here it is being used by itself, without a noun, so it means others or other people.

So grammatically, Alii = others / some other people.

Because it comes from alius and is in the masculine nominative plural.

The plural ending is effectively added so that the result is alii. In writing, that gives you two i’s in a row.

Learners often notice this because it looks unusual in English, but it is normal in Latin. In careful pronunciation, the two i sounds may be heard separately.

Because in can take two different cases:

• in + ablative = in / on / at a place, showing location
• in + accusative = into / onto a place, showing motion toward

Here the sentence says the action happens in the library, not that someone is moving into the library, so Latin uses the ablative:

• in bibliotheca = in the library
• in bibliothecam would mean into the library

Bibliotheca is ablative singular.

It is a first-declension noun, and after in with the meaning of location, first-declension nouns usually appear in the ablative singular ending -a.

So:

• nominative singular: bibliotheca
• ablative singular: bibliotheca

Those two forms happen to look the same in first-declension nouns, so you identify the case from the syntax, especially from the preposition in.

Because libros is the direct object of legunt.

The verb legunt means they read, and the direct object is the thing being read. In Latin, direct objects normally go in the accusative case.

So:

• libri = books as a subject
• libros = books as a direct object

Here they are reading the books, so Latin uses libros.

Because veteres agrees with libros.

Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

Libros is:

• masculine
• plural
• accusative

So the adjective must also be masculine plural accusative:

• libros veteres = old books

If it were vetera, that would be neuter plural, so it would not match libros.

No. Latin word order is much more flexible than English word order.

Because the endings show the grammar, Latin can place the adjective before or after the noun:

• libros veteres
• veteres libros

Both can mean old books.

The version in your sentence, libros veteres, is a very normal and natural order.

Legunt is the 3rd person plural present active indicative of legere.

That means:

• 3rd person plural = they
• present = happening now / generally
• active = the subject performs the action
• indicative = a normal statement

So legunt means they read or they are reading, depending on context.

The ending -unt is the important clue that the subject is they.

Yes. Latin often omits subject pronouns and even subject words when the verb ending already makes the subject clear.

Since legunt already means they read, a sentence like

In bibliotheca libros veteres legunt

could still be understood as They read old books in the library.

But adding Alii makes the subject explicit and can suggest a contrast, such as others as opposed to some different group.

No. Latin word order is flexible because the endings carry much of the grammatical information.

This sentence could be rearranged in several ways without changing the basic meaning, for example:

• Alii in bibliotheca libros veteres legunt
• Libros veteres alii in bibliotheca legunt
• In bibliotheca alii libros veteres legunt

The meaning stays roughly the same, but the emphasis may shift. Latin often puts the verb near the end, as in this sentence, but that is a tendency, not a strict rule.

Because Classical Latin does not have articles like English the and a/an.

So libros veteres can mean:

• old books
• the old books
• sometimes even some old books

The exact sense depends on context. Latin leaves that unstated unless something else in the sentence makes it specific.

Latin marks plurality very clearly in both the noun and the adjective.

Here:

• libros = accusative plural
• veteres = accusative plural

English also shows plural in books, but adjectives like old do not change. In Latin, adjectives do change, so both words show that the object is plural.

That is why Latin learners need to pay attention not just to the noun ending, but also to the adjective ending.