In piazza vedo una coppia che cammina lentamente.

In piazza is a very common Italian expression meaning in the square / at the town square in a general sense.

• in piazza = in the square, out in the square, at the square area
• nella piazza = in the square, but referring to a more specific, clearly identified square

So:

• In piazza vedo... sounds natural if you mean while I’m in the square or out in the square, I see...
• Nella piazza vedo... is possible, but it sounds more specifically tied to that particular square

Italian often uses these article-less place expressions in a way that English learners have to get used to.

Italian often drops subject pronouns when they are not needed.

• vedo already means I see
• the ending -o tells you the subject is I

So:

• (Io) vedo una coppia... = I see a couple...

Including io is possible, but it is usually only added for emphasis, contrast, or clarity:

• Io vedo una coppia, tu vedi un uomo solo.
  I see a couple, you see a man alone.

In a normal sentence, vedo by itself is enough.

They are both in the present tense.

vedo

This comes from vedere = to see

Present tense:

• io vedo = I see
• tu vedi = you see
• lui/lei vede = he/she sees

cammina

This comes from camminare = to walk

Present tense:

• io cammino = I walk
• tu cammini = you walk
• lui/lei cammina = he/she walks

So in the sentence:

• vedo = I see
• cammina = walks / is walking

Italian present tense can often correspond to both English simple present and present progressive, depending on context.

Yes, coppia is a feminine singular noun, so it takes the feminine singular article una.

• una coppia = a couple

Even though a couple consists of two people, the noun itself is grammatically singular:

• la coppia = the couple
• una coppia = a couple

This is why the verb later is also singular:

• una coppia che cammina
  literally: a couple that walks / is walking

Because the subject of cammina is una coppia, and coppia is grammatically singular.

Even though the idea involves two people, Italian agrees the verb with the noun coppia, not with the individuals inside the group.

So:

• una coppia che cammina = a couple who is walking / that is walking

Compare:

• Vedo una coppia che cammina.
  The grammatical subject is coppia → singular verb

But if you said:

• Vedo due persone che camminano.
  Due persone is plural → camminano

This is a very common point for English speakers, because English can sometimes feel more flexible with collective ideas.

Here che is a relative pronoun. It means who, that, or which, depending on the English translation.

In this sentence:

• una coppia che cammina lentamente
• literally: a couple that walks slowly
• more natural English: a couple who is walking slowly

So che connects una coppia to the extra information cammina lentamente.

It does not change form for gender or number here:

• l’uomo che parla = the man who is speaking
• la donna che parla = the woman who is speaking
• le persone che parlano = the people who are speaking

Lentamente is an adverb meaning slowly, and in Italian adverbs often come after the verb.

So:

• cammina lentamente = walks slowly / is walking slowly

This is the most neutral, natural placement.

Italian word order can sometimes vary for emphasis, but verb + adverb is the standard pattern here.

Compare:

• parla piano = speaks softly / quietly
• corre velocemente = runs quickly
• cammina lentamente = walks slowly

Yes. You could say:

• In piazza vedo una coppia che sta camminando lentamente.

This uses stare + gerundio, which is the Italian form closest to the English is walking.

Difference:

• che cammina lentamente = more neutral, very common
• che sta camminando lentamente = emphasizes the action as ongoing right now

In many cases, Italian simply uses the present tense where English would prefer is walking. So the original sentence is completely natural.

The original order is the most natural:

• In piazza vedo una coppia che cammina lentamente.

But Italian word order is somewhat flexible.

For example, you could say:

• Vedo una coppia che cammina lentamente in piazza.

This can also be correct, but it may slightly change the focus. The original version starts by setting the scene: In piazza...

You could also include io for emphasis:

• In piazza io vedo una coppia che cammina lentamente.

That is grammatical, but less neutral.

So yes, word order can change, but the original sentence is a very normal, natural way to say it.

Often coppia does mean a romantic couple, especially in a sentence like this. But the word itself can also mean simply pair or couple in other contexts.

Examples:

• una coppia di amici = a pair of friends
• una coppia di scarpe = a pair of shoes
• una coppia by itself often suggests a romantic couple

So in this sentence, without extra context, most people would probably understand a couple in the romantic sense.

Yes. Italian in does not always match English in word-for-word.

Depending on context, in piazza can be translated as:

• in the square
• at the square
• out in the square
• sometimes even on the square, depending on the exact English phrasing

The important thing is to understand it as a natural Italian location expression, not to translate the preposition too mechanically.