Philosophus dicit libros Graecos Romam saepe vehi.

Because Latin is using an indirect statement after dicit (says).

After verbs like dicit, Latin often does not use a separate word like that plus a finite verb. Instead, it uses:

• an accusative noun as the subject of the reported statement
• an infinitive as the verb of that statement

So:

• Philosophus dicit = The philosopher says
• libros Graecos ... vehi = that Greek books are carried / transported ...

In this construction, libros Graecos is the subject of the infinitive, but Latin puts that subject in the accusative.

So even though English treats Greek books as the subject of are carried, Latin uses the accusative because of the accusative-and-infinitive construction.

Yes. This is a classic Latin indirect statement.

The pattern is:

• main verb of saying/thinking/perceiving
• accusative + infinitive

Here:

• Philosophus dicit = The philosopher says
• libros Graecos Romam saepe vehi = that Greek books are often carried to Rome

So the whole sentence works as:

The philosopher says that Greek books are often transported to Rome.

Latin does not need a separate word for that here.

Vehi is the present passive infinitive of veho, vehere, vexi, vectum.

So:

• vehere = to carry, to transport (active infinitive)
• vehi = to be carried, to be transported (passive infinitive)

Because the books are receiving the action, Latin uses the passive infinitive:

• libros Graecos ... vehi = Greek books to be carried / that Greek books are carried

In smoother English, that usually becomes:

that Greek books are often transported to Rome

Because the books are not doing the carrying; they are the things being transported.

Compare:

• libros Graecos vehere = to carry Greek books
  
  • here someone unspecified is carrying the books
• libros Graecos vehi = for Greek books to be carried / that Greek books are carried
  • here the books are receiving the action

Since the meaning is about books being brought to Rome, the passive is the natural choice.

Romam is accusative because it expresses motion toward a place: to Rome.

With names of cities (and also small islands, plus domus, rus, and humus in certain uses), Latin often omits a preposition and simply uses the accusative to mean to that place.

So:

• Romam = to Rome
• Athenas = to Athens
• domum = homeward / to home

That is why Latin says Romam rather than ad Romam.

Normally, with the name of a city, classical Latin prefers the bare accusative:

• Romam = to Rome

Using ad with a city name is generally not the standard classical way in a sentence like this. So Romam is exactly what you would expect.

Because Graecos is an adjective modifying libros.

• libros = books
• Graecos = Greek

They agree in:

• case: accusative
• number: plural
• gender: masculine

So together they mean:

Greek books

Latin adjectives must agree with the nouns they describe.

Because liber (book) is a masculine noun in Latin.

So when Graecus, -a, -um is used to describe libri, it must take the masculine form:

• singular: liber Graecus
• plural accusative: libros Graecos

Even though in English Greek does not change form, Latin adjectives do.

Saepe means often, and it modifies the verbal idea vehi:

• Romam saepe vehi = to be often carried to Rome

So it tells you how frequently the transporting happens.

In English, we would usually say:

Greek books are often transported to Rome.

Latin word order is fairly flexible, so saepe could be placed in different positions without changing the basic meaning.

Here it comes before vehi, which is a very natural place for an adverb:

• libros Graecos Romam saepe vehi

This neatly keeps saepe close to the verb it modifies.

But Latin could rearrange the words for emphasis. For example, forms like these would still be understandable:

• Philosophus dicit saepe libros Graecos Romam vehi
• Philosophus dicit libros Graecos saepe Romam vehi

The exact placement can affect emphasis or rhythm more than core meaning.

The sentence is:

Philosophus dicit libros Graecos Romam saepe vehi.

A helpful breakdown is:

• Philosophus = subject of the main verb
• dicit = main verb
• libros Graecos = subject of the infinitive, in the accusative
• Romam = destination
• saepe = adverb
• vehi = infinitive completing the indirect statement

So the structure is roughly:

The philosopher says [Greek books to Rome often to-be-carried].

That sounds strange in English, but it is normal Latin.

The sentence does not say.

Because vehi is passive, Latin tells us what happens to the books, but not who transports them. The agent is left unspecified.

So the meaning is simply:

The philosopher says that Greek books are often transported to Rome.

If Latin wanted to say who was doing it, it could add an agent, usually with ab + ablative in a passive construction.

It can sometimes have a broader sense such as to ride, to go by conveyance, or to be conveyed.

But with books as the subject, the natural meaning is clearly:

to be carried / to be transported

So here you should understand:

Greek books are often transported to Rome

not Greek books often travel to Rome, even though that would be a playful literal English rendering.

Because Philosophus is the subject of the main verb dicit.

• Philosophus = the philosopher
• dicit = says

So:

The philosopher says ...

It is in the nominative singular because it is the subject of the finite verb in the main clause.

Yes. A very literal translation would be:

The philosopher says Greek books often to be carried to Rome.

That sounds unnatural in English, but it reflects the Latin structure.

A normal English translation is:

The philosopher says that Greek books are often transported to Rome.

So when reading Latin, it often helps to understand both:

• the literal structure
• the natural English meaning