Medicus dicit puerum mox sanum futurum esse.

Because this sentence uses an indirect statement after dicit. In Latin, indirect statement is usually expressed with an accusative + infinitive construction.

So the subject of the reported idea is not nominative; it becomes accusative. That is why Latin has puerum rather than puer.

A helpful comparison is:

• Direct statement: Puer mox sanus erit.
• Indirect statement after dicit: Medicus dicit puerum mox sanum futurum esse.

English often says The doctor says that the boy will soon be well. Latin usually does not use a separate word corresponding to English that in this kind of sentence.

Instead, Latin lets the accusative + infinitive construction do that job:

• dicit = says
• puerum mox sanum futurum esse = the reported statement

So the whole infinitive phrase functions like an English that-clause.

Futurum esse is the future infinitive of sum.

It is used here because the reported state of being well is still in the future relative to dicit. In smoother English, it corresponds to will be.

Literally, it is something like to be going to be, but that is not how we translate it naturally. The important point is that it marks futurity inside indirect statement.

Because erit is a finite verb, and in this construction Latin normally does not use a finite verb for the reported statement. After dicit, Latin switches to indirect statement, which uses an infinitive instead.

So:

• Direct: Puer mox sanus erit
• Indirect: puerum mox sanum futurum esse

In other words, erit becomes futurum esse when the statement is reported indirectly.

Because sanum agrees with puerum.

It is a predicate adjective describing the boy inside the indirect statement, and predicate adjectives agree with the noun they describe in gender, number, and case.

Since puerum is masculine singular accusative, sanum must also be masculine singular accusative.

For the same reason: futurum agrees with puerum.

Here futurum is masculine singular accusative, matching puerum. It is not neuter here, even though beginners often associate -um with neuter. In the second declension, -um can also be the masculine accusative singular.

So both sanum and futurum match puerum.

It would be:

Puer mox sanus erit.

Then, after a verb like dicit, Latin changes it into indirect statement:

• puer → puerum
• sanus → sanum
• erit → futurum esse

That is a very useful pattern to recognize.

Because esse by itself is the present infinitive, not the future infinitive.

So:

• puerum sanum esse = that the boy is well
• puerum sanum futurum esse = that the boy will be well

The sentence needs the second meaning, so Latin uses futurum esse.

The main finite verb is dicit.

Everything after that, puerum mox sanum futurum esse, is the content of what the doctor says. So when parsing the sentence, it helps to divide it into:

• Main clause: Medicus dicit
• Indirect statement: puerum mox sanum futurum esse

Mox is an adverb meaning soon.

It modifies the future idea: the boy will soon be well. Its position here is natural, but Latin word order is flexible, so an adverb like mox can often be moved without changing the basic meaning.

No. Medicus is the subject of dicit, not of futurum esse.

The understood subject of futurum esse is puerum, because in an accusative-and-infinitive construction, the accusative noun serves as the subject of the infinitive.

So:

• Medicus = the one doing the saying
• puerum = the one who will be well