Magistra dicit codicem in scrinio diligenter servandum esse.

Because after dicit in this sentence, Latin uses an indirect statement construction, often called the accusative-and-infinitive.

So instead of saying something like The teacher says that the book must be kept, Latin says, more literally:

• Magistra dicit
• codicem ... servandum esse

In this pattern:

• the subject of the reported statement goes into the accusative
• the verb of the reported statement goes into the infinitive

So codicem is accusative because it is the logical subject of servandum esse inside the indirect statement.

Servandum esse is a combination of:

• servandum = the gerundive of servare
• esse = to be

Together they form a passive periphrastic infinitive, which expresses necessity or obligation.

So servandum esse means:

• to have to be kept
• must be kept
• is to be kept

This is stronger than a plain passive infinitive such as servari (to be kept). The gerundive adds the idea of must or should.

Because servandum agrees with codicem.

The word codicem is:

• masculine
• singular
• accusative

So the gerundive must match those same features:

• masculine
• singular
• accusative

That gives servandum.

If the noun were feminine singular accusative, you would expect servandam.
If it were masculine singular nominative, you would expect servandus.

In Latin, the idea of must is often built into grammar rather than expressed by a separate word.

Here, the sense of obligation comes from the gerundive + esse construction:

• servandum esse = must be kept

So Latin does not need a separate word like English must. The obligation is already contained in the form.

In scrinio is a prepositional phrase meaning location.

• in with the ablative often means in or inside
• scrinio is ablative singular

So in scrinio tells you where the thing is to be kept.

It modifies servandum esse: the codex is to be kept in the desk / chest / case / writing box.

Because in takes different cases depending on meaning:

• in + accusative = motion into
• in + ablative = position in / on / within

Here there is no movement; it is about location. The codex is being kept in something, not moved into it. So Latin uses in scrinio, with the ablative.

Diligenter is an adverb, meaning something like carefully.

It modifies the verbal idea servandum esse:

• codicem ... diligenter servandum esse

So it tells us how the codex is to be kept: carefully.

It does not modify dicit here. The natural sense is not the teacher carefully says, but the teacher says that the codex must be carefully kept.

Because after verbs of saying, thinking, knowing, perceiving, and similar verbs, classical Latin very often uses the accusative-and-infinitive construction instead of a that-clause.

English says:

• The teacher says that ...

Latin commonly says:

• Magistra dicit
  • accusative + infinitive

So codicem ... servandum esse is the standard way to express the content of what the teacher says.

Not directly, even though it is accusative.

It belongs to the indirect statement after dicit. In other words, codicem is the subject of servandum esse, not the direct object of dicit in the ordinary sense.

A useful way to think of it is:

• Magistra dicit = main clause
• codicem in scrinio diligenter servandum esse = reported statement

So codicem is grammatically accusative because of the indirect statement, but logically it is the thing that must be kept.

Yes, but the meaning would change.

• servari = to be kept
• servandum esse = must be kept / ought to be kept

So servari would only express passive action, while servandum esse expresses necessity.

That distinction is important here.

Latin often leaves that unstated in a gerundive construction, especially in an indirect statement like this.

The sentence tells us that the codex has to be kept carefully in the desk, but it does not explicitly say by whom.

If Latin wanted to name the responsible person, it could use a dative of agent with the passive periphrastic. But here the focus is simply on the necessity of keeping the codex safely.

Because Latin word order is much more flexible than English word order.

The infinitive esse often comes near the end of an indirect statement, and the main idea can be spread out across the sentence:

• codicem
• in scrinio
• diligenter
• servandum esse

This arrangement is very natural in Latin. English usually keeps related words closer together, but Latin relies more on endings than on strict word order.

A helpful breakdown is:

• Magistra = subject of the main verb
• dicit = main verb
• codicem = accusative subject of the indirect statement
• in scrinio = where it is to be kept
• diligenter = how it is to be kept
• servandum esse = infinitive phrase expressing necessity

So the sentence consists of:

1. a main clause: Magistra dicit
2. an indirect statement: codicem in scrinio diligenter servandum esse

That is the key pattern to recognize.