In cista magna epistulae veteres servantur.

Because it follows in in the sense of location: in the box/chest.

In Latin, in can take:

• the ablative when it means in, on, inside somewhere
• the accusative when it means into somewhere

So in in cista magna, cista is ablative singular because the sentence describes where the letters are kept, not motion into the box.

Magna agrees with cista.

Both words are:

• feminine
• singular
• ablative

So in cista magna means in a large box/chest.

A very common Latin pattern is that adjectives must agree with the nouns they describe in:

• gender
• number
• case

Even if word order changes, that agreement tells you which words belong together.

You know from the form of the words and from the verb.

• epistulae here is nominative plural
• veteres matches it as nominative plural feminine
• servantur is third person plural

So epistulae veteres is the plural subject: old letters.

The verb agrees with that subject in number.

Yes. Epistulae can have more than one possible form depending on context. It could be:

• nominative plural = letters
• genitive singular = of a letter
• dative singular = to/for a letter

But in this sentence, it must be nominative plural because:

• it matches veteres
• it fits as the subject of servantur
• the meaning of the sentence requires letters as the thing being kept

This is very common in Latin: you often identify case by context, not just by the ending alone.

Veteres is the nominative plural feminine form of vetus, meaning old.

It matches epistulae:

• epistulae = feminine plural
• veteres = feminine plural

It is not veterae because vetus is a third-declension adjective, and third-declension adjectives do not use first/second-declension endings like -a, -ae in the same way that adjectives such as magnus, magna, magnum do.

So:

• magna comes from magnus
• veteres comes from vetus

Different adjective types, different endings.

Servantur is the present passive indicative, third person plural, of servare.

So it means:

• they are kept
• they are preserved
• sometimes, depending on context, they are being kept

Here it refers to epistulae veteres, so the meaning is old letters are kept/preserved.

Breakdown:

• serva- = verb stem
• -ntur = third person plural passive ending

That -ntur ending is a very useful signal that the verb is passive.

Latin often uses the passive when the focus is on the thing affected, not on who does the action.

Here the important idea is the condition or location of the letters: the old letters are kept in a large box.

If you wanted an active version, you would need a subject doing the keeping, for example:

• aliquis epistulas veteres in cista magna servat = someone keeps the old letters in a large box

The passive lets Latin leave the doer unstated.

Latin word order is much more flexible than English word order because Latin uses endings to show grammatical function.

English depends heavily on position:

• The old letters are kept in a large box

Latin can move parts around more freely:

• In cista magna epistulae veteres servantur
• Epistulae veteres in cista magna servantur
• Servantur in cista magna epistulae veteres

All of these can mean roughly the same thing, though the emphasis may shift.

In this sentence, starting with in cista magna puts the location first.

Classical Latin has no articles.

So cista can mean:

• a box
• the box

And epistulae veteres can mean:

• old letters
• the old letters

You decide from context which English translation sounds best.

This is one of the most important differences between Latin and English.

Yes.

With in:

• ablative = in/on a place, with no motion
• accusative = into a place, with motion toward it

Examples:

• in cista = in the box
• in cistam = into the box

So in this sentence, in cista magna uses the ablative because the letters are already there.

No, because the forms do not match.

• cista magna: both are ablative singular feminine
• epistulae veteres: both are nominative plural feminine

Since Latin adjectives must agree with their nouns in gender, number, and case, the pairings are clear.

So:

• magna goes with cista
• veteres goes with epistulae

That agreement helps you read Latin even when the word order is unfamiliar.