In bibliotheca codex veterrimus in cista magna servatur.

Latin word order is much more flexible than English word order. English usually needs something like The very old codex is kept..., with the subject near the beginning. Latin does not depend on word order as heavily, because the endings show the grammatical roles.

So In bibliotheca codex veterrimus in cista magna servatur is perfectly normal Latin. Starting with in bibliotheca puts the setting first: in the library.

A Latin speaker could also say:

• Codex veterrimus in bibliotheca in cista magna servatur
• In cista magna codex veterrimus in bibliotheca servatur

These would all be grammatical, though the emphasis would shift a little.

Because the sentence gives two pieces of location information:

• in bibliotheca = in the library
• in cista magna = in a large chest

So the codex is located in the library, and more specifically in a large chest.

Latin often stacks location phrases like this, just as English can: In the library, in a large chest, the very old codex is kept.

After in, Latin uses different cases depending on the meaning:

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

Here there is no movement. The codex is not being moved into the library or into the chest; it is already there. So Latin uses the ablative:

• in bibliotheca
• in cista magna

If there were movement, you would expect accusative forms instead, such as in bibliothecam or in cistam magnam.

Because codex is the subject of servatur.

The verb servatur means is kept or is preserved, so the thing being described as being kept is codex veterrimus. That makes it the subject, and subjects are normally in the nominative case.

So:

• codex veterrimus = the subject
• servatur = the verb
• in bibliotheca / in cista magna = location phrases

Veterrimus is the superlative form of vetus, which means old.

This is an irregular-looking superlative:

• vetus = old
• vetustior = older
• veterrimus = oldest / very old

A learner may expect a more regular superlative ending built directly on the adjective, but vetus has this special form.

Depending on context, a Latin superlative can mean either:

• a true superlative: the oldest
• an intensive superlative: very old

So codex veterrimus can mean the oldest codex or a very old codex, depending on the context and the meaning provided.

Because magna agrees with cista.

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

• gender
• number
• case

Here:

• cista is feminine, singular, ablative
• so the adjective must also be feminine, singular, ablative
• therefore: magna

So in cista magna means in a large chest.

Because its subject, codex veterrimus, is singular.

Servatur is:

• 3rd person
• singular
• present
• passive
• indicative

So it means is kept or is preserved.

If the subject were plural, the verb would also be plural. For example:

• codices veterrimi servantur = the very old codices are kept

The passive focuses on the thing being acted upon, not on the person doing the action.

• Active: aliquis codicem servat = someone keeps/preserves the codex
• Passive: codex servatur = the codex is kept/preserved

In your sentence, the important thing is the codex and where it is kept, not who keeps it. That is why the passive is a natural choice.

They match in gender, number, and case:

• codex = masculine, singular, nominative
• veterrimus = masculine, singular, nominative

That tells you veterrimus describes codex.

Latin allows words that belong together to be separated, but here they are next to each other, which makes the connection especially clear.

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

So a noun like codex can mean:

• a codex
• the codex

and cista magna can mean:

• a large chest
• the large chest

You decide from context which English article is best. That is why translation into English requires a choice even though Latin does not explicitly mark it.

Yes. In practice, these prepositional phrases are adverbial: they tell you the circumstances of the action servatur.

So they answer questions like:

• Where is it kept? — in bibliotheca
• In what place/container is it kept? — in cista magna

At the same time, in English we naturally understand them as describing where the codex is. So it is fine to think of them as giving the location of the codex, but grammatically they function as prepositional phrases connected with the whole clause.