Magistra et discipula una in bibliotheca sedent.

Because both nouns are feminine singular nominatives.

• magistra = teacher (female teacher)
• discipula = student or pupil (female student)

In this sentence, both are the subjects of the verb, so they appear in the nominative case. Many first-declension feminine nouns have -a in the nominative singular.

Because the subject is plural.

Even though there are two separate singular nouns, magistra et discipula together mean the teacher and the student, so Latin uses a plural verb:

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

So sedent matches the two people named in the sentence.

• magistra: nominative singular
• discipula: nominative singular
• bibliotheca: ablative singular

The first two are nominative because they are the subjects.
bibliotheca is ablative because it follows in to show location: in the library.

Because in takes different cases depending on the meaning.

• in + ablative = in / on somewhere, showing location
• in + accusative = into / onto somewhere, showing movement toward

Here the teacher and student are already sitting in the library, so Latin uses in bibliotheca.

If the sentence meant they go into the library, then you would expect in bibliothecam.

Here una means together.

It is being used as an adverb, describing how they are sitting: they are sitting together in the library.

This can confuse learners because una can also be the feminine form of one. But in this sentence it is not describing a noun. Instead, it tells us in what way they are sitting, so it means together.

No. In this sentence, una is not an adjective modifying either noun. It is an adverb.

So it does not mean one female teacher or one female student. It means together, and it applies to the whole action of sitting.

A good way to see this is that there are already two people in the sentence, so one would not make sense here.

Because that is a very common Latin word order.

Latin often puts the verb at or near the end, especially in straightforward sentences. So:

• Magistra et discipula una in bibliotheca sedent

is a very natural arrangement.

However, Latin word order is more flexible than English word order because the endings show the grammatical roles. So other orders are also possible, depending on emphasis.

Yes, often they could.

For example, Latin could also say:

• Discipula et magistra in bibliotheca una sedent
• In bibliotheca magistra et discipula sedent
• Una magistra et discipula in bibliotheca sedent

These would still basically mean the same thing, though the emphasis might change slightly. Latin uses endings, not just position, to show who is doing what.

Because Latin does not have articles like English.

English distinguishes between:

• a teacher
• the teacher

Latin usually just says magistra, and the exact meaning depends on context. So a noun in Latin can sometimes be translated as:

• a teacher
• the teacher
• simply teacher

The same is true for discipula and bibliotheca.

Usually, yes. In this sentence, et simply joins the two subjects:

• magistra et discipula = the teacher and the student

It is the normal Latin word for and.

Because they are both in the nominative case and are connected by et.

That tells us they form a compound subject. Then the plural verb sedent confirms that both people are sitting.

So the structure is:

• magistra et discipula = the two subjects
• sedent = what they are doing

Yes, it is.

sedent comes from the Latin verb sedere, meaning to sit. English words like sedentary come from the same root and refer to sitting a lot or not being physically active.

So seeing a familiar English derivative can help you remember the Latin verb.