Prologus primum actum aperit.

Because prologus is in the nominative singular, the case Latin normally uses for the subject.

A few clues help:

• prologus ends in -us, a very common nominative singular ending for a masculine second-declension noun
• aperit is third person singular, so it matches a singular subject
• if prologus were the direct object, you would expect prologum, not prologus

So prologus is the prologue as the thing doing the action.

Because they belong together as a noun phrase and are both in the accusative singular.

• actum is the direct object of aperit
• primum means first and modifies actum
• Latin adjectives agree with the nouns they describe in gender, number, and case

So:

• actum = accusative singular
• primum = accusative singular masculine, agreeing with actum

Together they mean the first act as the thing being opened.

Here it is best understood as an adjective.

Why?

• it matches actum in form
• it clearly describes which act
• so primum actum means the first act

It is true that primum can also be an adverb in other sentences, meaning something like first or at first. But in this sentence, because it agrees with actum, it is functioning as an adjective.

Aperit is:

• third person singular
• present tense
• active voice
• indicative mood

It comes from aperire, meaning to open.

So aperit means he/she/it opens or sometimes is opening, depending on how you choose to translate the Latin present tense into English.

Here the subject is prologus, so aperit means opens.

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

English depends heavily on position:

• subject + verb + object

Latin depends much more on endings, so the words can move around more freely without changing the basic meaning.

Putting the verb last is very common in Latin, especially in simple statements. So Prologus primum actum aperit is a very natural Latin order.

If the words were rearranged, the meaning could stay the same, though the emphasis might shift.

Because the first act is the object, not the subject.

Compare:

• primus actus = nominative singular, so it would mean the first act as a subject
• primum actum = accusative singular, so it means the first act as a direct object

In this sentence, the prologue is doing the action, and the first act is receiving it. So the accusative is required.

It comes from actus, which is a fourth-declension masculine noun.

That can surprise English-speaking learners, because actum looks a bit like a second-declension form. But here the noun is:

• nominative singular: actus
• accusative singular: actum

So actum is the correct accusative singular form of actus.

No. Classical Latin normally does not use articles like the or a/an.

So:

• prologus can mean the prologue or a prologue
• primum actum can mean the first act

Which English article to use depends on context and natural English style. In this sentence, English usually prefers the prologue and the first act.

Latin adjectives can come before or after the nouns they modify.

So both of these are possible in principle:

• primum actum
• actum primum

The choice often depends on style, rhythm, or emphasis rather than a strict rule like in English.

Here primum actum is simply a normal and clear way to say the first act.

A classroom Classical pronunciation would be roughly:

PRŌ-lo-gus PRĪ-mum AK-tum A-pe-rit

A few helpful points:

• prologus is stressed on the first syllable: PRŌlogus
• primum is stressed on PRĪ
• actum is stressed on AK
• aperit is stressed on the first syllable: Aperit

So the whole sentence sounds roughly like:

PRŌlogus PRĪmum AKtum APerit