Depois do filme, nós compramos sorvete e sentamos perto da escada rolante.

Because do is the normal contraction of de + o in Portuguese.

• depois de = after
• o filme = the movie
• depois de o filme becomes depois do filme

This kind of contraction is extremely common:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

So depois do filme is the natural, standard form.

It is a time expression meaning after the movie.

• depois = after / afterward
• depois de = after
• do filme = the movie

So the whole phrase sets the time for the rest of the sentence: first the movie happened, and then we bought ice cream and sat near the escalator.

Yes, it could be left out.

Portuguese often drops subject pronouns when the verb form already makes the subject clear. So both of these work:

• Nós compramos sorvete e sentamos perto da escada rolante.
• Compramos sorvete e sentamos perto da escada rolante.

Including nós can do a few things:

• add emphasis
• make the subject extra clear
• sound a little more explicit

So nós is correct, but not required.

Yes. For -ar verbs, the first-person plural form is the same in the present and the preterite.

With comprar:

• nós compramos = we buy
• nós compramos = we bought

Context tells you which one it means.

In this sentence, the time expression depois do filme strongly suggests a completed past event, so compramos is understood as we bought.

Yes. The same pattern happens with sentar.

• nós sentamos can mean we sit
• nós sentamos can mean we sat down

Again, context decides. Here, because the sentence is describing a sequence of past events after the movie, sentamos is understood as past: we sat down / we sat.

Not necessarily. In Brazilian Portuguese, sentar is often used without the reflexive pronoun in everyday speech.

So these can both exist:

• sentamos = we sat / we sat down
• nos sentamos = we sat down

In Brazil, the version without nos is very common and natural in conversation.

A useful idea:

• sentar can mean to sit or to sit down
• the reflexive form sentar-se also exists, but it often sounds a bit more formal or careful

So sentamos perto da escada rolante is perfectly normal Brazilian Portuguese.

Because Portuguese often leaves out the article when talking about something in a general, non-specific way.

So:

• compramos sorvete = we bought ice cream

This sounds natural if the idea is simply that you bought some ice cream, not a specifically identified ice cream.

You could also hear:

• compramos um sorvete = we bought an ice cream / an ice cream cone / one ice cream item
• compramos o sorvete = we bought the ice cream

In your sentence, sorvete without an article is completely natural.

Because perto normally takes the preposition de.

So the structure is:

• perto de = near

Then de + a escada rolante contracts to da escada rolante.

Step by step:

• perto de
• a escada rolante
• perto da escada rolante

So da appears for the same reason as do earlier: it is a contraction.

Literally, it is something like rolling staircase.

• escada = staircase / stairs
• rolante = rolling

But as a fixed expression, escada rolante simply means escalator.

This is the normal everyday Brazilian Portuguese word for escalator.

The comma separates the introductory time phrase from the main part of the sentence.

• Depois do filme, = After the movie,
• nós compramos sorvete... = we bought ice cream...

This comma is very natural and helps readability. In informal writing, some people might omit it, but using it here is correct and standard.

Because the sentence is narrating a sequence of completed past actions.

The structure is:

1. Depois do filme = after the movie
2. nós compramos sorvete = we bought ice cream
3. e sentamos perto da escada rolante = and sat near the escalator

Using the same tense for both verbs shows that these are two events that happened in sequence in the past.

Yes, somewhat. Portuguese allows some flexibility.

For example, these are all possible:

• Depois do filme, nós compramos sorvete e sentamos perto da escada rolante.
• Nós compramos sorvete e sentamos perto da escada rolante depois do filme.
• Compramos sorvete e sentamos perto da escada rolante depois do filme.

But the original version is very natural because it puts the time phrase first and sets the scene immediately.

The accent mark shows both pronunciation and meaning.

• nós = we
• nos = us / ourselves, as an object pronoun

So the accent helps distinguish two different words.

Examples:

• nós compramos = we bought
• ele nos viu = he saw us

That accent is important.