Se eu gastar muito hoje, não vou economizar para as férias.

Because Portuguese usually uses the future subjunctive after se when talking about a possible future condition.

So:

• Se eu gastar muito hoje... = If I spend a lot today...
• This is about a future possibility.

If you say Se eu gasto, that sounds unnatural in standard Brazilian Portuguese for this meaning.

A very common pattern is:

• Se + future subjunctive, main clause in the future / near future / imperative

Examples:

• Se eu tiver tempo, vou ligar.
• Se chover, ficamos em casa.

In your sentence, gastar is the future subjunctive form.

For many verbs, the future subjunctive looks identical to the personal infinitive or the plain infinitive in some forms.

With gastar:

• infinitive: gastar
• future subjunctive, eu form: gastar

So even though it looks like the infinitive, in Se eu gastar, it is functioning as the future subjunctive.

Some other forms make this easier to see:

• se nós gastarmos
• se eles gastarem

Those clearly are not just the plain infinitive.

Vou economizar is the near future construction:

• vou
  • infinitive

It means I am going to save or I won’t save in this sentence because of não.

So:

• não vou economizar = I’m not going to save

In Brazilian Portuguese, this structure is extremely common in everyday speech, often more common than the simple future.

Yes. Não economizarei is grammatically correct and means the same thing: I will not save.

But there is a difference in tone:

• não vou economizar = more common, natural, conversational
• não economizarei = more formal, more written, or more emphatic in some contexts

In everyday Brazilian Portuguese, vou + infinitive is usually the safer, more natural choice.

Yes, you can drop it:

• Se eu gastar muito hoje, não vou economizar para as férias.
• Se gastar muito hoje, não vou economizar para as férias.

Both are possible.

Portuguese often drops subject pronouns because the verb form already gives information about the subject. However, eu may still be used for:

• clarity
• emphasis
• contrast

So eu here is not required, but it sounds perfectly natural.

Not necessarily. Gastar can appear without a stated object if the context already makes it clear that we mean spend money.

So:

• Se eu gastar muito hoje... naturally suggests If I spend a lot today...

You could make it more explicit:

• Se eu gastar muito dinheiro hoje...

But it is not necessary. Omitting dinheiro is very normal.

Here, muito means a lot.

In this sentence it is working adverbially, modifying gastar:

• gastar muito = to spend a lot

Compare:

• muito dinheiro = a lot of money
• gastar muito = to spend a lot

So the same word can correspond to much, a lot, or many, depending on context.

Because the meaning is for the vacation / for the holidays, not during the vacation.

• economizar para as férias = save for vacation
• nas férias = on vacation / during the vacation period

So:

• Estou economizando para as férias. = I’m saving up for vacation.
• Nas férias, vou viajar. = During vacation, I’m going to travel.

Here, para expresses purpose.

In Portuguese, férias is normally used as a plural noun when it means vacation / time off.

So you say:

• as férias
• minhas férias
• nas férias

This is just how the word works in Portuguese, even when English uses singular vacation.

Also, in Brazilian Portuguese, férias often refers to a vacation period from work or school, not just a single trip.

Because férias normally takes the definite article:

• as férias = the vacation / vacation time

After para, the article stays:

• para as férias

This is very common in Portuguese. Articles are used more often than in English.

You will often see:

• viajar nas férias
• planejar as férias
• voltar das férias

In Portuguese, não normally goes directly before the verb or verb phrase it negates.

So:

• não vou economizar

This is the normal placement.

You would not say:

• vou não economizar in standard neutral Portuguese for this meaning

So the structure is:

• não + verb
• não + auxiliary + infinitive

Examples:

• não quero sair
• não vou viajar
• não posso pagar

It expresses a possible future condition.

The speaker is saying that one future action will affect another future result:

• If I spend a lot today, I won’t save for vacation.

This is not just a general truth; it is a specific future possibility. That is exactly why Portuguese uses the future subjunctive in the se clause.

Yes, in spoken Brazilian Portuguese, pra is very common and natural:

• Se eu gastar muito hoje, não vou economizar pras férias.

A couple of notes:

• pra = informal spoken form of para
• pras = contraction of pra + as

So all of these are possible depending on style:

• para as férias = neutral / standard
• pras férias = informal, very common in speech

Economizar is a very good choice and sounds natural. It means to save, especially in the sense of saving money or using less of something.

You might also hear:

• guardar dinheiro para as férias = save money for vacation
• juntar dinheiro para as férias = save up money for vacation

The differences are small:

• economizar often emphasizes being economical / not spending
• guardar dinheiro emphasizes setting money aside
• juntar dinheiro emphasizes accumulating money over time

In your sentence, economizar works very well because it contrasts nicely with gastar.

A natural Brazilian pronunciation would reduce some sounds in connected speech. Roughly:

Se eu gastar muito hoje, não vou economizar para as férias.

A few helpful notes:

• Se eu often sounds almost like seu in fast speech, but it is still se eu
• não vou often sounds like nãu vô
• para as may sound more like paras in natural speech
• férias has stress on the first syllable: FÉ-rias

A rough pronunciation guide:

• Se eu → seh EH-oo
• gastar → gas-TAR
• muito → MWIN-too
• hoje → OH-zhee
• não vou → now VOH
• economizar → eh-co-no-mee-ZAR
• férias → FEH-ree-as

The main stresses are:

• gasTAR
• MUIto
• HOje
• economiZAR
• FÉrias

Yes, a little, but the original order is the most neutral and natural.

Standard:

• Se eu gastar muito hoje, não vou economizar para as férias.

You could also say:

• Não vou economizar para as férias se eu gastar muito hoje.

That still sounds natural.

But the original version is often preferred because it presents the condition first and then the result. That is a very common structure in both Portuguese and English.