Il protagonista cambia, dunque l'arco narrativo cambia.

Because the definite article changes depending on the sound that follows.

• il is the masculine singular article used before most consonant sounds:
  il protagonista
• l' is used before singular nouns that begin with a vowel:
  l'arco

So l'arco is just il arco shortened by elision, which is required before a vowel.

The apostrophe shows that the vowel in il has dropped.

• full form: il
• before a vowel: l'

So:

• il protagonista
• l'arco

This is called elision. It helps the sentence sound smoother.

Because narrativo is an adjective agreeing with arco, and arco is masculine singular.

In Italian, adjectives usually agree with the noun they describe in gender and number.

• arco = masculine singular
• therefore narrativo = masculine singular

If the noun were feminine singular, the adjective would normally change too.

Dunque means something like therefore, so, or thus. It connects the first idea to the second and shows a result or conclusion.

So the structure is:

• Il protagonista cambia = first statement
• dunque = result/conclusion link
• l'arco narrativo cambia = consequence

It is a little more formal or literary than very everyday quindi or allora.

Yes, in many contexts you could.

• Il protagonista cambia, dunque l'arco narrativo cambia.
• Il protagonista cambia, quindi l'arco narrativo cambia.

Both are natural. The difference is mostly stylistic:

• dunque = slightly more formal, logical, or literary
• quindi = very common and neutral

Because there are two separate clauses, and each one has its own subject:

• Il protagonista cambia
• l'arco narrativo cambia

Italian normally repeats the verb in this kind of structure. Since the subjects are different, keeping both verbs makes the sentence clear and balanced.

In English, repetition might sometimes be avoided, but in Italian this repetition sounds normal.

Cambia is the third person singular present tense of cambiare.

So it can mean:

• changes
• is changing
• sometimes even a general truth like changes in a broad sense

Here it matches both singular subjects:

• il protagonista → singular
• l'arco narrativo → singular

That is why the verb is cambia in both clauses.

Italian often does not need subject pronouns, especially when the subject is already stated.

Here the subjects are explicit nouns:

• Il protagonista
• l'arco narrativo

So adding pronouns would be unnecessary. Italian usually prefers the noun alone unless there is a special reason for emphasis or contrast.

Because in Italian, many descriptive adjectives normally come after the noun.

So:

• arco narrativo

is the standard order.

Putting the adjective before the noun is possible with some adjectives, but it often changes the tone or is simply not natural in a specific expression. Arco narrativo is the normal fixed phrase.

Yes. Protagonista can refer to either a male or a female protagonist. The article tells you the gender:

• il protagonista = male protagonist
• la protagonista = female protagonist

So in your sentence, il suggests a male protagonist, or at least a masculine grammatical choice.

The comma separates two complete clauses and marks dunque as a connector between them.

• Il protagonista cambia
• dunque
• l'arco narrativo cambia

This punctuation is natural because dunque introduces the consequence of the first clause. Italian punctuation can be somewhat flexible, but the comma here is very normal and helps readability.

Not grammatically. It is not built as an if... then... sentence.

A true conditional would use something like:

• Se il protagonista cambia, l'arco narrativo cambia.

Your original sentence is different: it presents the second clause as a conclusion from the first, using dunque. So it is more like:

• The protagonist changes; therefore, the narrative arc changes.

Yes, but the original order is the most straightforward and neutral:

• Il protagonista cambia, dunque l'arco narrativo cambia.

Italian does allow other word orders for emphasis, but they may sound more marked or literary. For a learner, the original subject-verb order is the safest and most natural choice here.