Vou guardar a primeira fatia até que o Pedro chegue a casa.

Vou guardar is the very common ir + infinitive future, similar to I’m going to save/keep in English.

In European Portuguese, this structure is extremely common in everyday speech:

• Vou guardar = I’m going to keep/save
• Guardarei = I will keep/save

Both are correct, but:

• vou guardar sounds more natural and conversational
• guardarei sounds more formal, more literary, or more emphatic

So in normal spoken Portuguese, vou guardar is exactly what you would expect.

Here, guardar means to save or to keep for later.

It can also mean:

• to store
• to put away
• to keep
• to guard/protect in some contexts

In this sentence, it most naturally means something like:

• to save the first slice
• to keep the first slice aside

So it is not necessarily about physically locking something away; it is more about not letting anyone eat it yet.

Fatia means slice.

It is used for things that are cut into slices, such as:

• cake
• pizza
• bread
• ham
• cheese

So a primeira fatia means the first slice.

A useful comparison:

• fatia = slice
• pedaço = piece, chunk, bit

So fatia is more specific than pedaço.

Primeira is an ordinal adjective, meaning first, and in Portuguese it usually comes before the noun:

• a primeira fatia = the first slice
• o primeiro dia = the first day

Also notice the agreement:

• fatia is feminine singular
• so the article is a
• and primeiro becomes primeira

So:

• a primeira fatia = feminine singular noun phrase

In European Portuguese, it is very common to use the definite article before a person’s name:

• o Pedro
• a Maria
• o João

This often sounds very natural in Portugal.

For an English speaker, this can feel strange, because English does not normally say the Pedro. But in Portuguese this does not mean anything special here; it is just normal grammar and usage.

A few extra points:

• In European Portuguese, the article before names is very common.
• In Brazilian Portuguese, usage varies more, and the article is often omitted.
• You may still see Pedro without the article in some contexts, especially in more formal writing.

Até que means until when it is followed by a full clause with a conjugated verb.

Here the part after it is a clause:

• o Pedro chegue a casa

So:

• até que o Pedro chegue a casa = until Pedro gets home

A helpful contrast:

• até
  • noun/phrase: até amanhã = until tomorrow
• até que
  • clause: até que ele chegue = until he arrives

So até que is used because what follows is not just a noun or time expression, but a whole clause.

Because after até que, when you are talking about a future event that has not happened yet, Portuguese normally uses the present subjunctive.

So:

• até que o Pedro chegue a casa = until Pedro gets home

Here, Pedro has not arrived home yet. The arrival is still pending, so Portuguese uses the subjunctive.

That is why you get:

• chegue = present subjunctive not
• chega = present indicative

This is a very common pattern:

• Espero que ele venha.
• Vou esperar até que ela termine.
• Fico aqui até que o Pedro chegue.

For English speakers, this can feel odd because English usually just uses a normal present form in similar sentences.

This is a spelling change to keep the consonant sound of the verb stem.

The infinitive is:

• chegar

When the ending begins with e, Portuguese spelling changes g to gu so that the sound stays consistent:

• chegar
• chegue
• cheguem

If Portuguese wrote chege, the g would represent a different sound.

So the u is there mainly for spelling reasons: it helps preserve the consonant value of the verb.

Because with chegar, the normal expression is chegar a casa = to arrive home / get home.

Compare:

• chegar a casa = to arrive home
• estar em casa = to be at home
• ir para casa = to go home

So each one fits a different idea:

• a casa after chegar
• em casa for location
• para casa for movement toward home, especially with verbs like ir or voltar

In this sentence, Pedro is arriving, so chegue a casa is the natural choice.

Yes. That is also natural Portuguese.

Portuguese often allows two patterns here:

• até que + subjunctive
• até + infinitive (often a personal infinitive in other persons)

So both of these work:

• até que o Pedro chegue a casa
• até o Pedro chegar a casa

They mean essentially the same thing here.

A rough difference:

• até que o Pedro chegue a casa can sound a bit more explicit or slightly more formal
• até o Pedro chegar a casa is also very common and natural

Since Pedro is third person singular, the infinitive form is just chegar.

Yes, it has a couple of features that strongly fit European Portuguese usage.

The clearest one is:

• o Pedro

Using the article before a first name is especially typical of Portugal.

Also, the sentence overall sounds very natural in European Portuguese:

• vou guardar for the near future
• até que
  • subjunctive
• chegar a casa

None of this is wrong in Brazilian Portuguese, but the article before the name is one of the strongest clues pointing to European Portuguese.

The subject of chegue is o Pedro.

So the structure is:

• Vou guardar a primeira fatia = main clause
• até que o Pedro chegue a casa = subordinate clause

Inside the subordinate clause:

• o Pedro = subject
• chegue = verb
• a casa = destination/complement

So Pedro is the one who is doing the arriving.