In bibliotheca undecim discipuli et undecim discipulae hodie adsunt.

Because in bibliotheca means in the library in the sense of location.

With in, Latin uses:

• ablative for location: in bibliotheca = in the library
• accusative for motion toward: in bibliothecam = into the library

So here, since the students are already there, bibliotheca is ablative singular.

Because undecim (eleven) is an indeclinable numeral. That means its form does not change.

The nouns after it are simply put in whatever case the sentence requires. Here they are the subjects of the sentence, so they are in the nominative plural:

• discipuli = male students / boys
• discipulae = female students / girls

So:

• undecim discipuli = eleven male students
• undecim discipulae = eleven female students

Latin often distinguishes grammatical gender more clearly than English does.

• discipulus = a male student
• discipula = a female student

Their plural nominative forms are:

• discipuli = male students
• discipulae = female students

So the sentence is specifically saying there are eleven male students and eleven female students.

They are both nominative plural because they are the subjects of adsunt.

You can see this from their endings:

• discipulus → discipuli
• discipula → discipulae

Both are regular nominative plural forms.

Because the subject is plural: undecim discipuli et undecim discipulae.

In fact, the subject contains two groups joined by et (and), so a plural verb is required.

• singular: adest = is present
• plural: adsunt = are present

So adsunt matches the plural subject.

Adsunt comes from adsum, adesse, which literally means to be present, to be here, or to be at hand.

So:

• sunt = they are
• adsunt = they are present / they are here

In this sentence, adsunt is more natural than plain sunt because it emphasizes presence in a place.

Latin word order is more flexible than English word order. Hodie means today, and it can appear in different positions depending on style or emphasis.

So all of these are possible in Latin:

• Hodie undecim discipuli et undecim discipulae in bibliotheca adsunt.
• In bibliotheca hodie undecim discipuli et undecim discipulae adsunt.
• In bibliotheca undecim discipuli et undecim discipulae hodie adsunt.

The sentence you have is perfectly normal. Putting hodie near the end can make it feel like a small adverbial addition: are present today.

Classical Latin has no articles like English the or a/an.

So:

• bibliotheca can mean a library or the library
• discipuli can mean students or the students

The exact sense depends on context. In this sentence, English naturally supplies the in translation: in the library.

Yes. Latin word order is flexible, so you could rearrange the sentence without changing the basic meaning.

For example:

• Undecim discipuli et undecim discipulae hodie in bibliotheca adsunt.
• Hodie in bibliotheca undecim discipuli et undecim discipulae adsunt.

The original version begins with In bibliotheca, which puts the location first. That may help set the scene: In the library...

Et means and. It joins the two subject phrases:

• undecim discipuli
• undecim discipulae

So the full subject is eleven male students and eleven female students.

Because two subject groups are joined by et, the verb is plural: adsunt.

No. This is a very common point for learners.

With in:

• ablative = in / on with no movement
  
  • in bibliotheca = in the library
• accusative = into / onto with movement toward
  
  • in bibliothecam = into the library

So you have to ask: Are we describing where someone is, or where someone is going?

It is plural in meaning, even though the numeral itself does not show singular/plural endings.

Because undecim means eleven, it refers to more than one person, so the noun is plural:

• undecim discipuli
• not undecim discipulus

And the verb is plural too when that phrase is the subject.

Sometimes in Latin, a masculine plural can refer to a mixed group, depending on context. However, in this sentence the writer has chosen to mention both groups separately:

• discipuli = male students
• discipulae = female students

That makes the meaning explicit and balanced.

They are:

• in = in, into
• bibliotheca, bibliothecae f. = library
• undecim = eleven
• discipulus, discipuli m. = male student
• discipula, discipulae f. = female student
• et = and
• hodie = today
• adsum, adesse = to be present

Knowing the dictionary form helps you recognize why the sentence uses these particular endings.

A helpful way is to break it into parts:

• In bibliotheca = where?
• undecim discipuli et undecim discipulae = who?
• hodie = when?
• adsunt = what are they doing?

So the structure is:

[place] + [subject] + [time] + [verb]

That kind of breakdown is very useful when reading Latin, especially because the word order is freer than in English.