Pater dicit modum etiam in laetitia servandum esse.

Because Latin normally uses indirect statement after verbs of saying, thinking, knowing, and perceiving.

So instead of saying:

• Father says that ...

Latin says:

• Pater dicit ... esse

This pattern is called the accusative-and-infinitive construction.

In this sentence:

• Pater = the subject of the main verb, Father
• dicit = says
• modum ... servandum esse = the content of what he says

So Latin expresses that moderation must be preserved by using an infinitive phrase rather than a separate that-clause.

In an indirect statement, the subject of the infinitive is usually put in the accusative.

Here, modum is the thing being talked about in the reported statement:

• modum servandum esse = that moderation must be preserved

Even though in English we would think of moderation as the subject of must be preserved, Latin puts it in the accusative because it is the subject of the infinitive phrase inside indirect statement.

So:

• modum = accusative singular of modus
• it is the accusative subject of servandum esse

Servandum esse is a form expressing necessity or obligation.

It is made of:

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

Together they mean something like:

• must be preserved
• ought to be maintained
• has to be kept

So:

• modum servandum esse = that moderation must be maintained

This is often called the passive periphrastic.

Because the gerundive must agree with the noun it goes with.

Here the noun is:

• modum = masculine singular accusative

So the gerundive also has to be:

• masculine singular accusative = servandum

If the noun were different, the gerundive would change too. For example:

• modi servandi = of modes/standards that must be preserved
• moda servanda would match a neuter plural noun

In this sentence, servandum matches modum exactly.

Here modus does not mean just manner or way in a broad sense. In this context it means something more like:

• moderation
• measure
• self-control
• restraint

So modum servare is an idiomatic Latin idea meaning:

• to keep moderation
• to show restraint
• to stay within proper limits

That is why the sentence means that one should remain moderate even when happy.

In laetitia means:

• in joy
• in happiness
• amid rejoicing

The noun laetitia is in the ablative because the preposition in takes the ablative when it means in, within, or during in a non-motion sense.

So:

• in + ablative = in / during / amid
• laetitia is ablative singular

Here it means that moderation should be preserved even in a state of joy or happiness.

Etiam means also, even, or still.

In this sentence it most naturally emphasizes in laetitia:

• even in happiness
• even in joy

Latin word order is flexible, so etiam is placed before the phrase it especially highlights. Here it stresses that moderation is necessary not only in difficult situations, but even when things are joyful.

Because Pater is the subject of the main verb:

• Pater dicit = Father says

Only the subject of the infinitive clause goes into the accusative in indirect statement.

So the structure is:

• Pater = nominative, subject of dicit
• modum = accusative, subject of servandum esse

This is a very common point of confusion for English speakers: not every subject becomes accusative, only the subject inside the reported statement.

Yes, Latin could say modum servari, but that would mean simply:

• that moderation is being preserved
• that moderation is preserved

That is only a plain passive infinitive.

But servandum esse adds the idea of necessity:

• that moderation must be preserved

So the sentence is not just reporting a fact. It expresses a principle or obligation.

Yes. The sentence does not state explicitly who must do it.

Modum servandum esse literally means:

• that moderation must be preserved

Latin often leaves the responsible person unstated when it is general or obvious. In English we might supply:

• one must preserve moderation
• people should keep moderation
• we should remain moderate

If Latin wanted to name the person responsible, it could use a dative of agent with this kind of construction.

Yes, but it is also very normal Latin word order.

The sentence goes:

• Pater — subject first
• dicit — main verb early
• modum etiam in laetitia servandum esse — the indirect statement after it

Within the indirect statement, Latin places the key noun modum first, then adds the phrase etiam in laetitia, and ends with servandum esse, the verbal idea of obligation.

Ending with esse is very common in Latin infinitive constructions, and placing servandum near the end gives emphasis to the idea of necessity.