Prologus ante primum actum dicitur.

Dicitur is the 3rd person singular present passive indicative of dico, dicere, which usually means to say.

So literally it means:

• it is said
• or, in this context, more naturally, it is spoken / it is recited

Because the subject is prologus (the prologue), the whole sentence means something like:

• The prologue is spoken before the first act.

In theatrical Latin, dico can mean not just say, but also deliver, recite, or speak aloud.

Prologus is the subject of the sentence, so it appears in the nominative singular.

The verb dicitur means is spoken / is said, and the thing that is spoken is the subject in a passive sentence:

• active idea: Someone speaks the prologue
• passive Latin: The prologue is spoken

So prologus is nominative because it is the grammatical subject of dicitur.

Because it comes after the preposition ante.

When ante is used as a preposition meaning before, it normally takes the accusative case. So:

• ante actum = before the act

Here we have:

• ante primum actum = before the first act

So actum is accusative because ante governs the accusative.

Because primum is an adjective describing actum, and adjectives in Latin must agree with the nouns they modify in:

• gender
• number
• case

Here:

• actus is masculine
• singular
• accusative after ante, so it becomes actum

Therefore primus must also become masculine singular accusative:

• primus → nominative singular masculine
• primum → accusative singular masculine

So:

• primum actum = the first act

Actum is the accusative singular of actus, a 4th-declension masculine noun.

Its basic forms are:

• nominative singular: actus
• genitive singular: actus
• accusative singular: actum

In this sentence it means act in the theatrical sense, as in a division of a play.

Latin normally has no definite article and no indefinite article.

So prologus can mean:

• a prologue
• the prologue

and primum actum can mean:

• a first act
• the first act

You decide from context which English translation sounds best. In this sentence, the prologue and the first act are the most natural translations.

Yes, Latin could rearrange them more freely than English.

This sentence is:

• Prologus ante primum actum dicitur.

But Latin could also say things like:

• Ante primum actum prologus dicitur.
• Dicitur prologus ante primum actum.

The basic meaning would stay the same because the cases show the grammatical relationships:

• prologus = subject
• ante primum actum = prepositional phrase
• dicitur = verb

That said, the given order is perfectly natural and clear.

Because in Latin, the verb form itself often includes the idea of is, was, are, and so on.

Dicitur already means:

• he/she/it is said
• it is spoken

So Latin does not need a separate word like English is in this sentence.

Here ante means before in time.

So ante primum actum means:

• before the first act

In other contexts, ante can also mean in front of in a spatial sense, but here the theatrical context makes the time meaning obvious.

It is a Latin word, but it is originally borrowed from Greek.

That is very common in literary and theatrical vocabulary. Even though it comes from Greek, Latin treats it like a normal second-declension masculine noun:

• nominative singular: prologus
• accusative singular: prologum
• etc.

So for a learner, the important thing is that it behaves like an ordinary masculine noun in -us.

In some sentences, yes: dicitur can mean is called or is said to be called, depending on context.

But in this sentence, that translation does not fit well. The sentence is about when the prologue is delivered in a play:

• The prologue is spoken/recited before the first act

So here dicitur is best understood as is spoken, is said, or is recited, not is called.

The sentence breaks down like this:

• Prologus — subject: the prologue
• ante primum actum — prepositional phrase: before the first act
• dicitur — verb: is spoken / is recited

So the structure is:

• Subject + prepositional phrase + verb

A very literal translation would be:

• The prologue before the first act is spoken

But smoother English is:

• The prologue is spoken before the first act.