Philosophus dicit libros Graecos ad Romam saepe vehi.

Because philosophus is the subject of the main verb dicit.

• philosophus = the philosopher
• nominative singular masculine
• main-clause subject: The philosopher says...

So this part is straightforward: philosophus dicit = the philosopher says.

In this sentence, Latin uses indirect statement, also called the accusative-and-infinitive construction.

Instead of saying something like The philosopher says that... with a separate word for that, Latin normally does this:

• verb of saying/thinking/perceiving
• then an accusative subject
• plus an infinitive

So:

• dicit = says
• libros Graecos ... vehi = that Greek books are carried

Latin often leaves out any separate word corresponding to English that in this kind of sentence.

Because in an indirect statement, the subject of the infinitive goes into the accusative.

So although Greek books are logically the subject of vehi (to be carried), Latin puts them in the accusative because they are the subject of an infinitive in indirect statement.

• libros = accusative plural of liber
• Graecos = accusative plural of Graecus
• together: Greek books

This is one of the most important Latin patterns to learn:

• English: He says that the books are carried
• Latin: He says the books to be carried

That sounds unnatural in English, but it is normal Latin structure.

Vehi is the present passive infinitive of veho, meaning to carry, to transport, or to convey.

So vehi means:

• to be carried
• to be transported
• sometimes more naturally in English, to be brought

In this sentence, libros Graecos ... vehi means Greek books to be carried / that Greek books are carried.

Because the sentence is focusing on what happens to the books, not on who carries them.

• active: someone carries the books
• passive: the books are carried

Latin often uses the passive when the agent is unknown, unimportant, or simply not mentioned.

So the sentence does not say who transports the books. It only says that this happens.

Vehi is a present infinitive. In indirect statement, the present infinitive usually shows action happening at the same time as the main verb.

Since dicit is present (says), vehi suggests something like:

• says that Greek books are often carried to Rome
• or says that Greek books are often being carried to Rome

In normal English, are often carried is usually the best translation.

Because Latin word order is more flexible than English word order.

Graecos is an adjective modifying libros, and it agrees with it in:

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

So:

• libros Graecos = Greek books

Latin could also say Graecos libros. Both are possible. The ending shows the relationship more than the position does.

Ad with the accusative means to or toward.

So:

• ad Romam = to Rome

Romam is accusative because it is the object of the preposition ad.

This phrase shows motion toward a place, which is why the accusative is used.

That is a very good question.

Yes, with names of cities, Latin often uses the bare accusative without a preposition to mean to a city:

• Romam = to Rome

So you might expect libros Graecos Romam saepe vehi.

However, ad Romam is also possible. It can give a slightly stronger sense of direction such as:

• to Rome
• toward Rome
• to the area of Rome

In many teaching sentences, ad + accusative is used because it is easy to recognize as a motion-toward phrase. But you are right that bare Romam is the more classic city-name construction.

Saepe means often, and it modifies vehi.

So the sense is:

• Greek books are often carried to Rome

Its placement is flexible. Latin adverbs can often appear in several places without changing the basic meaning. Here saepe sits just before vehi, which makes it easy to connect often with the action to be carried.

Yes. Latin word order is quite flexible because the endings show the grammatical roles.

For example, these would express basically the same idea:

• Philosophus dicit libros Graecos ad Romam saepe vehi.
• Philosophus libros Graecos ad Romam saepe vehi dicit.
• Libros Graecos philosophus dicit ad Romam saepe vehi.

The exact emphasis may shift a little, but the grammar remains clear because of the forms:

• philosophus = nominative subject of dicit
• libros Graecos = accusative subject of the infinitive
• vehi = passive infinitive

A natural translation is:

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

You could also say:

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

The best English choice depends on how literal you want to be, but carried or brought is usually fine.