Marta e Paolo si fidanzano.

Because the subject is Marta e Paolo, which means two people. In Italian, two coordinated singular nouns make a plural subject, so the verb has to be third person plural.

• lui/lei si fidanza = he/she gets engaged
• loro si fidanzano = they get engaged

So Marta e Paolo si fidanzano uses the same verb form as loro si fidanzano.

Because the verb is fidanzarsi, not just fidanzare, when it means to get engaged / to become a couple.

Here, si is a clitic pronoun that goes with the verb. With a plural subject like Marta e Paolo, it often has a reciprocal sense: they get engaged to each other.

So in this sentence, si does not mean a separate word like English themselves in a literal way; it is part of how the verb is used.

The dictionary form is fidanzarsi.

That is the infinitive:

• fidanzarsi = to get engaged / to become boyfriend and girlfriend, depending on context

The sentence uses the present-tense form:

• (loro) si fidanzano

In a normal statement with a finite verb, Italian clitic pronouns usually go before the verb:

• si fidanzano
• si sposano
• si vedono

With an infinitive, it attaches to the end:

• fidanzarsi
• sposarsi
• vedersi

So si fidanzano is the normal word order in a sentence like this.

In this sentence, it is best understood as each other.

With two or more people as the subject, Italian often uses the same reflexive form for a reciprocal meaning:

• Marta e Paolo si amano = Marta and Paolo love each other
• Marta e Paolo si fidanzano = Marta and Paolo get engaged to each other

So although the form is reflexive, the meaning here is reciprocal.

It is the present indicative.

More specifically:

• fidanzano = third person plural, present tense
• si fidanzano = they get engaged / they are getting engaged

Italian uses the present tense in several ways, depending on context:

• for something happening now
• for a general fact
• sometimes for a near future event
• sometimes in storytelling

So the exact English translation can vary with context, even though the Italian form stays the same.

Yes, it can, especially if there is a time expression.

For example:

• Marta e Paolo si fidanzano domani.

That can mean Marta and Paolo are getting engaged tomorrow.

Without extra context, Marta e Paolo si fidanzano is just the present form, but Italian often uses the present for planned future events.

This is a very important difference.

• si fidanzano = they become engaged / they start being a couple
• sono fidanzati = they are engaged / they are a couple

So:

• Marta e Paolo si fidanzano focuses on the change or the event
• Marta e Paolo sono fidanzati focuses on the state after that

A rough English parallel is:

• they get engaged vs.
• they are engaged

Not always. This is a nuance learners should know.

In modern Italian, fidanzato / fidanzata can mean:

• fiancé/fiancée
• but also boyfriend/girlfriend, depending on context

So fidanzarsi can sometimes mean:

• to get engaged in a formal sense
• or more loosely to become a couple

The exact force depends on context, age, region, and situation.

Not for this meaning.

To mean Marta and Paolo get engaged, you need fidanzarsi, so you need si:

• Marta e Paolo si fidanzano.

Without si, the verb would be understood differently, and it would not normally mean that they become engaged to each other.

So for this sentence, si is necessary.

You can say:

• Marta si fidanza con Paolo.

That means Marta gets engaged to Paolo / Marta starts going out with Paolo, depending on context.

But if both people are the subject, Italian very naturally says:

• Marta e Paolo si fidanzano.

That is often the most natural way to express the idea when both names are mentioned.

Because personal names in standard Italian normally appear without an article:

• Marta
• Paolo
• Marta e Paolo

So Marta e Paolo si fidanzano is completely normal.

In some regional varieties of Italian, especially in parts of northern Italy, you may hear articles before names in speech, but that is not the neutral standard pattern.

Yes. e means and.

So:

• Marta e Paolo = Marta and Paolo

It simply joins the two names into one compound subject, which is why the verb becomes plural.