Io e Marta ci conosciamo da due anni.

Both are possible.

• Io e Marta = Marta and I
• Marta e io = Marta and I

In Italian, putting io first is common in everyday speech, even though some teachers may prefer Marta e io because it sounds more polite or formal, similar to English.

So this sentence is completely natural:

• Io e Marta ci conosciamo da due anni.

And this is also correct:

• Marta e io ci conosciamo da due anni.

You can usually leave it out if the subject is already clear.

Italian often drops subject pronouns, because the verb ending shows who the subject is. However, in this sentence, io e Marta is not just the pronoun io—it is the full subject Marta and I. So you do need some version of the subject.

You could say:

• Io e Marta ci conosciamo da due anni.
• Marta e io ci conosciamo da due anni.

But you cannot just say:

• Ci conosciamo da due anni.

unless the context already makes it clear who we are.

Ci is the reciprocal pronoun here. It means something like each other.

So:

• conosciamo = we know
• ci conosciamo = we know each other

That is why the sentence means:

• Marta and I have known each other for two years

Without ci, conosciamo would usually need a direct object:

• Conosciamo Marta = We know Marta
• Conosciamo Roma = We know Rome

But in ci conosciamo, the action goes both ways: I know Marta, and Marta knows me.

It is often called reflexive, but more precisely here it is reciprocal.

• reflexive = the subject does something to itself
• reciprocal = two or more people do something to each other

In this sentence:

• ci conosciamo does not mean we know ourselves
• it means we know each other

So grammatically it uses the same kind of pronoun as reflexive verbs, but the meaning is reciprocal.

This is a very common Italian-English difference.

Italian often uses the present tense for actions or states that started in the past and are still true now, especially with da.

So:

• Ci conosciamo da due anni. literally looks like:
• We know each other for two years but the natural English translation is:
• We have known each other for two years.

Italian does this a lot:

• Abito qui da tre mesi. = I have lived here for three months.
• Studio italiano da un anno. = I have been studying Italian for a year.

Da due anni means for two years in this sentence.

With a present tense verb, da + time expression often shows:

• something began in the past
• and is still continuing now

So:

• da due anni = for two years

Examples:

• Lavoro qui da due anni. = I have worked here for two years.
• Siamo amici da due anni. = We have been friends for two years.

Because da is the normal choice when something started in the past and continues into the present.

Compare:

• Ci conosciamo da due anni. = We have known each other for two years. = it started two years ago and is still true now

But per due anni usually refers to a completed duration, often in the past:

• Ci siamo frequentati per due anni. = We dated for two years. = possibly finished now

So in your sentence, da is the correct preposition because the relationship of knowing each other still continues.

Italian usually does not use an article in time expressions like this.

So you say:

• da due anni = for two years
• da tre giorni = for three days
• da un mese = for a month

This is normal and natural. You do not need i or gli here.

No, not with the meaning of the original sentence.

Ci conosciamo da due anni means:

• We have known each other for two years

But siamo conosciuti is not the normal way to express that idea.

You might see:

• Ci siamo conosciuti due anni fa. This means:
• We met each other two years ago.

So there is an important difference:

• Ci conosciamo da due anni = we have known each other for two years
• Ci siamo conosciuti due anni fa = we met two years ago

These are related ideas, but not the same.

The infinitive is conoscersi.

The verb changes like this:

• conoscersi = to know each other / to get to know each other, depending on context

In the present tense:

• mi conosco
• ti conosci
• si conosce
• ci conosciamo
• vi conoscete
• si conoscono

In your sentence, ci conosciamo is the first person plural form:

• we know each other

In theory, the form could look similar, but in real life this sentence is understood as we know each other.

With two people as the subject, ci conosciamo is naturally interpreted as reciprocal:

• I know Marta, and Marta knows me

If you wanted to express the idea of we know ourselves, that would usually need a clearer context, because it is not the normal interpretation here.

No, Italian word order is fairly flexible.

These are all possible:

• Io e Marta ci conosciamo da due anni.
• Marta e io ci conosciamo da due anni.
• Da due anni io e Marta ci conosciamo.

The most neutral and natural version is the original one. Moving da due anni to the front can add a little emphasis to the duration.

Not naturally in this exact structure.

If you want to say Marta and I, you normally keep both parts of the subject:

• Io e Marta ci conosciamo da due anni.

Italian does not usually use a subject pronoun combination like lei ed io in ordinary speech unless you want emphasis or contrast:

• Lei e io ci conosciamo da due anni.

That is grammatically possible, but it sounds more marked and less everyday than using the person’s name.

Italian uses conoscere for knowing a person, a place, or being familiar with something.

So:

• conoscere una persona = to know a person
• conoscere una città = to know a city

Sapere is used for facts, information, or how to do something:

• So la risposta. = I know the answer.
• So nuotare. = I know how to swim.

Since the sentence is about two people being acquainted with each other, conoscere is the correct verb:

• Io e Marta ci conosciamo da due anni.