Undecim puellae in bibliotheca sedent.

Because puellae is plural nominative: girls as the subject of the sentence.

The singular is puella = girl.
The plural nominative is puellae = girls.

Since the girls are the ones doing the action of sitting, Latin uses the nominative case.

Because its subject, undecim puellae, is plural.

sedet = he/she/it sits
sedent = they sit

Latin verbs change form to match the subject in person and number, so with eleven girls, the verb must be plural.

No. Undecim = eleven, and it is indeclinable, which means its form does not change.

So you get:

• undecim puellae = eleven girls
• undecim servos = eleven slaves
• undecim urbes = eleven cities

The noun changes as needed, but undecim stays the same.

Because in can take either the ablative or the accusative, depending on the meaning.

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

Here the sentence means the girls are sitting in the library, so it is location, not movement. Therefore Latin uses the ablative:

• in bibliotheca = in the library

If it meant into the library, it would be:

• in bibliothecam

It is ablative singular.

The basic form is bibliotheca. For a first-declension noun like this, the ablative singular is also bibliotheca, so it looks the same as the nominative singular.

You know it is ablative here because it follows in in a location sense: in bibliotheca.

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

So puellae can mean:

• girls
• the girls

And in bibliotheca can mean:

• in a library
• in the library

Which one is best depends on context. English must choose an article, but Latin usually does not.

Yes. Latin word order is much freer than English word order because the endings show the grammar.

So all of these could mean essentially the same thing:

• Undecim puellae in bibliotheca sedent
• In bibliotheca undecim puellae sedent
• Sedent undecim puellae in bibliotheca

The original order is perfectly normal, but Latin can move words around for emphasis or style.

Because the Latin verb already includes the subject information.

sedent by itself means they sit. In many contexts, that is enough without a separate pronoun or helping verb.

English often needs more words, but Latin packs more information into the verb ending.

Yes. Latin present tense often covers both English:

• they sit
• they are sitting

So sedent can be translated either way depending on context.

The dictionary form is sedeō, sedēre, sēdī, sessum = sit.

sedent is the third-person plural present active indicative form, meaning they sit or they are sitting.

Yes, though it is borrowed from Greek.

bibliotheca means library or book-room. It is treated as a regular first-declension feminine noun in Latin.

So a learner can decline it like other first-declension nouns, such as puella.

Because the verb ending already tells you the subject is third-person plural.

• sedent = they sit

Latin often leaves pronouns out unless they are needed for emphasis or contrast. If you added eae or another pronoun, it would usually sound more emphatic: they sit.

Normally, no. In Latin, a numeral like undecim with a plural noun such as puellae takes a plural verb, because the subject is grammatically plural.

So undecim puellae sedent is the expected pattern.

The subject is undecim puellae = eleven girls.

More specifically:

• undecim modifies the noun
• puellae is the noun in the nominative plural
• together they form the full subject

The predicate is in bibliotheca sedent = are sitting in the library.