A professora pediu-nos que ficássemos no recreio até que o refeitório abrisse.

In European Portuguese, object pronouns are very often attached to the end of the verb with a hyphen. That is called enclisis.

So:

• pediu-nos = asked us

Here, nos means us.

In Brazilian Portuguese, nos pediu is more common in everyday speech, but in Portugal pediu-nos sounds very natural and standard.

So:

• A professora pediu-nos... = The teacher asked us...

Nos is the pronoun meaning us.

The verb pedir often works like this:

• pedir a alguém que... = to ask someone to...

In the sentence:

• A professora pediu-nos que ficássemos...

the nos tells you who was asked: us.

So the structure is basically:

• A professora = the teacher
• pediu-nos = asked us
• que ficássemos... = that we stay / to stay...

Because after pedir que, Portuguese normally uses the subjunctive.

So:

• pedir que + subjunctive

Here the teacher made a request, so Portuguese uses the subjunctive to express what was requested.

Since the main verb is in the past (pediu), Portuguese uses the imperfect subjunctive:

• ficássemos = imperfect subjunctive of ficar

That is why you get:

• pediu-nos que ficássemos

and not:

• pediu-nos que ficávamos
• pediu-nos que ficamos

Those would sound wrong here.

This is largely a matter of sequence of tenses.

The main verb is in the past:

• pediu = asked

The staying in the playground was something the teacher wanted to happen at that past time / after that past moment, so Portuguese normally uses the imperfect subjunctive in the subordinate clause.

A useful pattern is:

• present/main idea → often present subjunctive
• past/main idea → often imperfect subjunctive

Compare:

• A professora pede-nos que fiquemos no recreio.
  The teacher asks us to stay in the playground.

• A professora pediu-nos que ficássemos no recreio.
  The teacher asked us to stay in the playground.

Because até que often triggers the subjunctive when it refers to something that had not happened yet at that point.

In this sentence, the opening of the dining hall was still a future event from the perspective of the teacher’s request. So Portuguese uses:

• até que ... abrisse

Here abrisse is the imperfect subjunctive of abrir.

So the idea is:

• stay in the playground until the dining hall opened / should open

not as a simple factual event already completed, but as a pending event in the future relative to that past request.

Yes, that structure is also possible in Portuguese:

• até o refeitório abrir

This uses the personal infinitive / infinitive construction rather than até que + subjunctive.

Both can express until the dining hall opened / until the dining hall should open.

However, the sentence you were given uses the very common pattern:

• até que + subjunctive

That pattern is especially useful for learners because it clearly shows that the action was still pending.

Yes. Here ficar means to stay, to remain, or to wait in a place.

So:

• ficássemos no recreio = that we stay/remain in the playground

In other contexts, ficar can mean other things too, such as:

• to become
• to be located
• to end up

But in this sentence, stay/remain is the right meaning.

No recreio means in the playground or in the schoolyard / at recess area, depending on context.

Grammatically:

• em + o = no

So:

• no recreio = in the playground / in the schoolyard

Also, recreio can refer not only to the physical place, but sometimes to the break/recess period. In this sentence, because of ficássemos no recreio, it most naturally sounds like the place: the playground or schoolyard area.

This is about contractions and articles.

1. no recreio

This comes from:

• em + o recreio = no recreio

So no already contains the article o.

2. o refeitório

Here there is no preposition before it, so the article appears on its own:

• até que o refeitório abrisse

So both nouns actually have the masculine singular article o:

• no recreio = in the playground
• o refeitório = the dining hall

Portuguese uses definite articles much more often than English does, especially with professions, known people, and specific nouns in context.

So:

• A professora literally = the teacher

In English, we might sometimes say Teacher asked us... in a school context, but Portuguese normally prefers the article here.

This is very natural:

• A professora pediu-nos...

It refers to a specific teacher already known in the situation.

Not exactly. Abrisse is not the simple past; it is the imperfect subjunctive form of abrir.

Here are some forms:

• abrir = to open
• abriu = opened (simple past / preterite)
• abria = was opening / used to open / opened, depending on context
• abrisse = opened / should open / were to open, in a subjunctive context

In this sentence, abrisse is used because it depends on até que in a past-time setting.

So:

• até que o refeitório abrisse

does not simply mean until the dining hall opened as a plain factual past tense form; it reflects the idea of a future event seen from a past point of view.

Yes. A common alternative would be something like:

• A professora pediu-nos para ficarmos no recreio até o refeitório abrir.

This is also understandable and natural.

Your original sentence is a bit more explicitly subjunctive-based:

• pediu-nos que ficássemos... até que... abrisse

That version is especially useful for learning because it shows two important patterns:

• pedir que + subjunctive
• até que + subjunctive

So even if other natural alternatives exist, the original sentence is a very good example of standard Portuguese grammar.

Yes, it has features that strongly fit European Portuguese.

The clearest one is:

• pediu-nos

That attached pronoun after the verb is very characteristic of standard European Portuguese.

A Brazilian Portuguese speaker would understand the sentence perfectly, but in Brazil it would be more common to hear something like:

• A professora nos pediu que ficássemos...
• or another reworded structure in everyday speech

So the sentence is fully correct Portuguese in general, but its style clearly leans toward Portugal.