Depois do pequeno-almoço, ficaram migalhas na mesa e no chão.

Because do is the contraction of de + o.

The expression is depois de + noun phrase, and pequeno-almoço normally takes the definite article o in Portuguese. So:

• depois de + o pequeno-almoço → depois do pequeno-almoço

This kind of contraction is very common:

• depois da aula = after the lesson
• depois do almoço = after lunch
• depois dos jogos = after the games

So do here does not mean anything extra by itself; it is just the normal contracted form.

Portuguese often uses the definite article with meal names where English does not.

So in European Portuguese, phrases like these are very natural:

• antes do pequeno-almoço
• depois do almoço
• durante o jantar

English often leaves the article out, but Portuguese does not always do that. This is one of those places where you should not translate word for word from English.

Because pequeno-almoço is a fixed compound noun in European Portuguese. It is the standard spelling for breakfast in Portugal.

A learner may notice that it looks literally like small lunch, but as a whole it simply means breakfast.

Also, this is very much a Portugal Portuguese word. In Brazilian Portuguese, the usual expression is café da manhã.

Because the subject is migalhas, which is plural.

Even though migalhas comes after the verb, it is still the subject:

• ficaram migalhas

So the verb must agree with migalhas:

• ficou migalha = one crumb remained
• ficaram migalhas = crumbs remained

This word order, with the verb before the subject, is very natural in Portuguese in sentences that present a result or describe what was left.

Ficar has several meanings, and here it means to remain, to be left, or to end up being somewhere.

So:

• ficaram migalhas na mesa e no chão

means something like:

• crumbs were left on the table and on the floor
• crumbs remained on the table and on the floor

It is not about the crumbs actively staying somewhere. It is describing the result after breakfast.

Ficaram is the pretérito perfeito (simple past / preterite). It presents the event as completed.

That fits this sentence well: after that breakfast, the result was that crumbs were left behind.

If you used ficavam, it would usually suggest something habitual, repeated, or backgrounded:

• Depois do pequeno-almoço, ficavam sempre migalhas na mesa. = After breakfast, there were always crumbs left on the table.

So:

• ficaram = one completed occasion
• ficavam = repeated or habitual situation

These are contractions of em with the definite articles:

• em + a → na
• em + o → no

So:

• na mesa = em a mesa
• no chão = em o chão

Portuguese uses these contractions constantly, and they are required in standard usage.

Yes, you could, but the nuance is slightly different.

• ficaram migalhas... emphasizes that the crumbs were left behind as a result of breakfast.
• havia migalhas... simply states that there were crumbs there.

So:

• Depois do pequeno-almoço, ficaram migalhas na mesa e no chão. = After breakfast, crumbs were left on the table and on the floor.

• Depois do pequeno-almoço, havia migalhas na mesa e no chão. = After breakfast, there were crumbs on the table and on the floor.

Both are correct, but ficaram is a bit more result-focused.

Yes. When talking about crumbs in a general sense, Portuguese usually uses the plural migalhas.

The singular migalha exists, but it usually refers to:

• one single crumb
• a tiny amount, sometimes figuratively

Examples:

• Havia uma migalha na mesa. = There was one crumb on the table.
• Havia migalhas na mesa. = There were crumbs on the table.

So in this sentence, the plural is the natural choice.

The grammatical subject is migalhas.

There is no hidden person doing the action in this sentence. The sentence is built to focus on the result, not on who caused it.

So the structure is:

• time phrase: Depois do pequeno-almoço
• verb: ficaram
• subject: migalhas
• place: na mesa e no chão

If you wanted to say who caused the crumbs, you would need a different sentence, for example:

• Depois do pequeno-almoço, as crianças deixaram migalhas na mesa e no chão.

That means the children left crumbs on the table and on the floor.