Na festa, meu sogro pediu um pedaço de bolo e milho com manteiga.

Na is a contraction of em + a.

• em = in / at / on
• a = the for a feminine singular noun

Because festa is feminine singular, em a festa becomes na festa.

Very common contractions:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

So na festa means at the party or in the party, depending on context.

The comma separates the opening time/place expression from the main clause.

• Na festa, = setting the scene first
• meu sogro pediu... = main information

In English, this is similar to At the party, my father-in-law ordered...

In Portuguese, that comma is very natural here. With a short opening phrase, some writers may omit it, but using it is clear and standard.

In Brazilian Portuguese, both forms are possible:

• meu sogro
• o meu sogro

Both mean my father-in-law.

In Brazil, leaving out the article before a possessive is very common, especially in everyday speech. Adding the article can sound a little more formal, regional, or stylistically marked, depending on the speaker.

So meu sogro pediu... is completely normal.

Pediu is the preterite form of pedir in the 3rd person singular.

• infinitive: pedir = to ask for / to order
• ele pediu = he asked for / he ordered

The preterite is used for a completed action in the past. Here, it shows that the request/order happened as a single finished event.

Compare:

• pediu = he asked for / ordered
• pedia = he was asking for / used to ask for

So pediu is the natural choice for a simple past event.

Um pedaço de bolo means a piece of cake, so it is more specific than just bolo.

• bolo = cake
• um pedaço de bolo = a piece of cake

If you said pediu bolo, that could mean he asked for some cake in a more general sense. But um pedaço de bolo emphasizes one portion.

Also, pedaço de is a very common structure in Portuguese:

• um pedaço de pizza
• um pedaço de pão
• um pedaço de queijo

It works much like English a piece of...

Because Portuguese normally links a measure/portion noun to another noun with de.

So:

• um pedaço de bolo = a piece of cake
• uma fatia de pão = a slice of bread
• um copo de água = a glass of water

Here, pedaço is the portion noun, and bolo is the thing being portioned. The de is required in this structure.

Portuguese often leaves out articles with food items when talking about them in a general, non-specific way, especially in contexts like eating, buying, or ordering.

So:

• pediu milho com manteiga = he ordered corn with butter

This sounds like the name of an item or dish, not necessarily a specifically identified corn and a specifically identified butter.

If you add articles, the meaning can become more specific:

• o milho = the corn
• a manteiga = the butter

But in this sentence, the article-less version sounds natural and idiomatic.

The natural reading is that com manteiga goes with milho only.

So the sentence is understood as:

• um pedaço de bolo and
• milho com manteiga

In other words, he asked for two items:

1. a piece of cake
2. corn with butter

Portuguese readers would not normally understand this as cake with butter and corn. The structure and common sense both point to butter modifying milho.

He is most naturally asking for two things.

The conjunction e joins:

• um pedaço de bolo
• milho com manteiga

So the sentence means he requested both items together.

If the speaker wanted to make that separation even clearer, they could say:

• meu sogro pediu um pedaço de bolo e um milho com manteiga
• or use a pause in speech

But the original sentence already strongly suggests two separate food items.

Yes. Portuguese allows some flexibility.

For example:

• Na festa, meu sogro pediu um pedaço de bolo e milho com manteiga.
• Meu sogro pediu um pedaço de bolo e milho com manteiga na festa.

Both are grammatical.

The version starting with Na festa puts the setting first. That is very common in narration because it tells you where the action happened before telling you what happened.

So the original sentence is a very natural way to present the information.