Magister dicit librum de magistratibus non celeriter legendum esse, quia multa in margine scribenda sint.

It breaks down like this:

• Magister dicit = The teacher says
• librum de magistratibus non celeriter legendum esse = an indirect statement
• quia multa in margine scribenda sint = a because-clause explaining the reason

So the skeleton is:

• The teacher says [that the book about magistrates should not be read quickly], because [many things must be written in the margin].

A Latin learner should especially notice that the part after dicit is not introduced by that. Latin usually uses an accusative + infinitive construction instead.

Because it is the subject of an indirect statement after dicit.

After verbs like dicit, putat, audit, scit, Latin often uses:

• accusative subject
• infinitive verb

So in:

• magister dicit librum ... legendum esse

the word librum is the grammatical subject of legendum esse, but because it is inside indirect statement, it appears in the accusative.

This can feel strange in English, because we would say:

• the teacher says that the book should be read

where the book looks like a normal subject. Latin does not do that here; it turns the subject into the accusative.

Legendum esse is a gerundive + esse construction, often called the passive periphrastic.

From lego, legere = to read, we get:

• legendus, -a, -um = to be read, needing to be read, must be read

So:

• librum legendum esse literally means that the book is to be read
• more naturally: that the book must be read or should be read

Because it is passive, Latin focuses on the thing that needs the action:

• the book must be read

rather than saying directly who must read it.

Because legere and legendum esse do not mean the same thing.

• legere = to read / to be reading
• legendum esse = to have to be read / should be read / must be read

So:

• magister dicit librum legere... would not make sense here as the teacher says the book to read
• magister dicit librum legendum esse correctly expresses necessity or obligation

The gerundive is the key part that adds the idea of must / should.

Because the passive periphrastic is made with:

• gerundive + a form of sum

In a normal finite clause, you might have:

• liber legendus est = the book must be read

But after dicit, Latin needs an infinitive, not a finite verb. So est becomes:

• esse

That is why we get:

• librum legendum esse

not librum legendum est.

De takes the ablative, so magistratibus is ablative plural.

• de magistratibus = about magistrates / concerning magistrates

This phrase modifies librum:

• librum de magistratibus = the book about magistrates

So de here does not mean down from; it means about, concerning.

Celeriter is an adverb meaning quickly. It modifies the verbal idea in legendum esse.

So:

• non celeriter legendum esse = should not be read quickly

Latin often uses a simple adverb like this where English might say:

• not quickly
• not too quickly
• slowly
• carefully (depending on context)

The literal Latin wording is simply not quickly.

Because multa here means many things.

It is the neuter plural of multus, -a, -um and is being used substantively, meaning the noun is understood rather than stated.

So:

• multa = many things
• multa scribenda = many things needing to be written

This is very common in Latin. Neuter plural adjectives are often used by themselves to mean things like:

• multa = many things
• pauca = a few things
• bona = good things

This is another gerundive construction, parallel to legendum esse.

From scribo, scribere = to write, we get:

• scribendus, -a, -um = to be written, must be written

Since multa is neuter plural, the gerundive must agree with it:

• multa scribenda

And because the subject is plural, the verb is plural too:

• sint = are in the subjunctive

So:

• multa in margine scribenda sint = many things must be written in the margin

Because this quia clause is being presented as part of the teacher’s reported reasoning.

In Latin, when a clause belongs to indirect discourse or reflects someone’s thought or explanation, subordinate clauses often go into the subjunctive.

So here:

• magister dicit ... quia multa in margine scribenda sint

means the reason is being given from the teacher’s point of view:

• the teacher says ... because many things have to be written in the margin

If Latin used sunt, that would sound more like the narrator himself was stating the reason as a plain fact. With sint, the sentence keeps the reason inside the teacher’s reported statement or viewpoint.

Margine is ablative singular after in.

With in, Latin distinguishes:

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

Here the meaning is location:

• in margine = in the margin / on the margin

If it meant motion into the margin, Latin would use something like in marginem.

Correct: the person responsible is left unstated.

That is normal with the gerundive/passive periphrastic. Latin can simply say:

• the book must be read
• many things must be written

without naming the person who must do it.

If Latin wanted to state the responsible person, it often used the dative of agent:

• discipulo librum legendum esse = that the student must read the book
• ei multa scribenda sunt = he has many things to write

In your sentence, the agent is omitted because it is either obvious from context or not important.

They are doing similar jobs:

• librum ... legendum esse = the book should/must be read
• multa ... scribenda sint = many things should/must be written

So the sentence has a neat internal parallel:

• one thing must be read
• many things must be written

That makes the logic very clear:

• the book should not be read quickly
• because there are many notes/comments to write in the margin

Seeing that parallel helps a lot when reading Latin prose.