In volumine veteri oratio de pace tam clara est ut etiam pueri eam intellegant.

Because in with a fixed location takes the ablative in Latin.

So:

• in volumine = in the book / in the volume
• veteri = old, and it agrees with volumine

Agreement means the adjective matches the noun in:

• case: ablative
• number: singular
• gender: neuter

So volumine veteri means in the old book.

Because vetus, veteris is a third-declension adjective, and here it must match volumine.

The noun volumine is:

• ablative singular
• neuter

So the adjective must also be ablative singular, which is veteri.

A learner often expects adjectives to behave like first/second-declension adjectives, but vetus belongs to the third declension, so its endings are different.

Because the preposition de takes the ablative.

So:

• de pace = about peace or concerning peace

This is just something to memorize with the preposition:

• de + ablative

Tam ... ut is a very common Latin pattern meaning so ... that.

In this sentence:

• tam clara est = is so clear
• ut etiam pueri eam intellegant = that even boys/children understand it

So the structure is:

• tam = so
• ut = that

This introduces a result clause: the speech is so clear that a certain result follows.

Because it is in a result clause introduced by ut after tam.

Latin normally uses the subjunctive in clauses like this:

• tam ... ut ...
• so ... that ...

So intellegant is not subjunctive because the action is doubtful; it is subjunctive because Latin grammar requires it in this construction.

Here, intellegant means understand in the clause that even children understand it.

Because it agrees with oratio.

• oratio is feminine singular nominative
• clara must match it

So:

• oratio ... clara est = the speech/discourse is clear

Even though English does not usually show this kind of agreement so visibly, Latin adjectives regularly match the nouns they describe.

Eam refers back to oratio.

That means:

• oratio = the speech/discourse
• eam = it, meaning the speech

It is feminine singular accusative because:

• it refers to oratio, which is feminine singular
• it is the direct object of intellegant

A very important Latin rule is that a pronoun agrees with its antecedent in gender and number, but its case depends on its job in its own clause.

So eam is feminine because of oratio, but accusative because it is the thing being understood.

Pueri is the nominative plural of puer, meaning boys or sometimes more generally children, depending on context.

Here it is nominative plural because it is the subject of intellegant:

• pueri intellegant = the boys/children understand

So in the result clause:

• etiam pueri = even boys / even children

Etiam means even or also, depending on context.

Here it means even:

• ut etiam pueri eam intellegant
• that even children understand it

It adds emphasis. The idea is that the speech is so clear that not only educated adults, but even children, can understand it.

Latin word order is much more flexible than English word order because Latin shows grammatical relationships through endings, not mainly through position.

So Latin can place words in an order that highlights emphasis or style.

For example:

• In volumine veteri is placed first to set the scene: in the old book
• oratio de pace gives the main subject: the speech about peace
• tam clara est gives the main statement: is so clear
• ut etiam pueri eam intellegant gives the result

A very literal English order would sound awkward, but the Latin order is natural and often stylistic.

Yes. De pace modifies oratio.

So:

• oratio = speech, oration, discourse
• de pace = about peace

Together:

• oratio de pace = the speech about peace

This is similar to English, where a prepositional phrase can describe a noun.

Yes, depending on context.

Volumen can mean:

• a scroll
• a book
• a volume

So in volumine veteri could be translated as:

• in the old book
• in the old volume
• in the old scroll

All are grammatically possible. The best choice depends on the larger context and the style of translation.