Tres discipuli in bibliotheca libros veteres legunt.

Tres is the masculine/feminine nominative plural form of the Latin numeral three.

It is tres here because it goes with discipuli (students), which is:

• masculine
• plural
• nominative (the subject of the sentence)

So tres discipuli means three students.

A learner may notice that Latin numerals can change form to match gender and case. For example, three students is tres discipuli, but three books would use the neuter form tria libri only if libri were neuter—but it is not; liber is masculine, so three books is also tres libri. The neuter form tria is used with neuter nouns.

Because discipuli is the subject of the verb legunt (they read / are reading).

The basic idea is:

• Who is doing the action? → tres discipuli
• Therefore, discipuli must be in the nominative case.

Discipulus is a second-declension masculine noun:

• singular nominative: discipulus = student
• plural nominative: discipuli = students

So tres discipuli legunt = three students read.

The preposition in can take either:

• the ablative for location: in / in(side), at
• the accusative for motion toward: into

Here, the meaning is in the library as a place where the students already are, not motion into it. So Latin uses the ablative:

• bibliotheca = library
• in bibliotheca = in the library

Compare:

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

That distinction is very important in Latin.

Because libros is the direct object of legunt.

Ask:

• What are the students reading?
• books

In Latin, the direct object usually goes in the accusative case.

Liber (book) is a second-declension masculine noun:

• nominative singular: liber
• accusative singular: librum
• nominative plural: libri
• accusative plural: libros

So libros means books as the thing being read.

Veteres is the adjective agreeing with libros.

In Latin, adjectives must agree with the nouns they describe in:

• gender
• number
• case

Here, libros is:

• masculine
• plural
• accusative

So the adjective must also be masculine plural accusative:

• veteres

That gives:

• libros veteres = old books

Even though the noun and adjective may belong to different declensions, they still have to match in gender, number, and case.

Because Latin word order is much more flexible than English word order.

In English, we normally say old books.
In Latin, both of these are possible:

• libros veteres
• veteres libros

Both mean old books.

Latin often places adjectives after nouns, especially in straightforward prose, but there is no single rigid rule like in English. Word order in Latin often reflects emphasis, style, or rhythm more than basic grammar.

So libros veteres is perfectly normal.

Legunt is the third person plural present active indicative of legere, meaning to read.

So legunt means:

• they read
• or they are reading

We know who is doing the action because the ending -unt tells us the verb is third person plural, and the explicit subject is tres discipuli.

A quick breakdown:

• lego = I read
• legis = you read
• legit = he/she/it reads
• legimus = we read
• legitis = you all read
• legunt = they read

So tres discipuli ... legunt = three students ... read / are reading.

Classical Latin does not normally use articles like English the and a/an.

So:

• discipuli can mean students, the students, or sometimes some students
• libros veteres can mean old books or the old books
• in bibliotheca can mean in a library or in the library

The exact choice in English depends on context and translation style.

That is why the Latin sentence can be translated in more than one natural way, even though the Latin itself does not include separate words for the or a.

Yes, it could be rearranged, and it would still mean essentially the same thing because the endings show the grammatical relationships.

This sentence is:

Tres discipuli in bibliotheca libros veteres legunt.

Possible rearrangements include:

• In bibliotheca tres discipuli libros veteres legunt.
• Libros veteres tres discipuli in bibliotheca legunt.
• Tres discipuli libros veteres in bibliotheca legunt.

All of these still mean roughly Three students are reading old books in the library.

However, different word orders can shift emphasis:

• putting in bibliotheca first emphasizes the location
• putting libros veteres first emphasizes the object
• putting legunt at the end is very common in Latin prose

So Latin word order is flexible, but not random.

Because the Latin present tense often covers both ideas:

• simple present: they read
• present progressive: they are reading

So legunt can mean either one, depending on context.

English often forces you to choose between:

• they read
• they are reading

Latin usually does not make that distinction in the same way. The context tells you which English translation sounds best.

In this sentence, are reading is often the most natural English translation, but read is grammatically possible too.