Depois da compra, eu vou guardar o comprovante na pasta azul.

Because da is the contraction of de + a.

• de = of / from / after in many contexts
• a = the feminine singular definite article the
• de + a = da

So depois da compra literally breaks down as after the purchase.

This kind of contraction is very common in Portuguese:

• de + o = do
• de + a = da
• em + a = na
• em + o = no

In Brazilian Portuguese, ir + infinitive is one of the most common ways to talk about the future.

So:

• eu vou guardar = I’m going to keep/store/put away
• A more formal or less common alternative would be guardarei

Both are correct, but eu vou guardar sounds much more natural in everyday Brazilian speech.

Compare:

• Eu vou guardar o comprovante. = very common, conversational
• Guardarei o comprovante. = correct, but more formal or written

Not always. Portuguese often drops subject pronouns when the verb already makes the subject clear.

So both of these work:

• Eu vou guardar o comprovante na pasta azul.
• Vou guardar o comprovante na pasta azul.

Including eu can add:

• emphasis
• clarity
• contrast

For example:

• Eu vou guardar o comprovante, mas ela vai jogar fora.

In your sentence, eu is perfectly natural, but it could be omitted.

Here, guardar means something like:

• to keep
• to store
• to put away
• to save in the physical sense

In this sentence, it suggests putting the receipt somewhere safe or designated.

It is not usually the same as computer-related save, which is often salvar:

• Vou salvar o arquivo. = I’m going to save the file.
• Vou guardar o comprovante. = I’m going to put away / keep the receipt.

So guardar is a very natural choice for a physical object like a receipt.

Comprovante is a very useful word in Brazilian Portuguese. It usually means some kind of proof or document that confirms something.

Depending on context, it can be:

• a receipt
• a proof of payment
• a voucher
• a confirmation slip
• some other document showing that something happened

In this sentence, after a purchase, comprovante most likely means a receipt or proof of purchase/payment.

Common examples:

• comprovante de pagamento = proof of payment
• comprovante de residência = proof of address
• comprovante de matrícula = proof of enrollment

Because na is the contraction of em + a.

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

So:

• na pasta azul = in the blue folder

Also, pasta is feminine:

• a pasta
• therefore na pasta

If the noun were masculine, you would use no:

• no carro = in the car

In Portuguese, adjectives often come after the noun.

So:

• pasta azul = blue folder
• literally folder blue

This is very normal word order in Portuguese.

You will see this a lot:

• carro vermelho = red car
• camisa branca = white shirt
• casa grande = big house

Some adjectives can come before the noun, but with colors, the usual order is after the noun.

Portuguese uses definite articles more often than English does.

So where English might say:

• I’m going to keep the receipt or even
• I’m going to keep receipt/documentation in some contexts,

Portuguese very naturally says:

• vou guardar o comprovante

The article o helps make the noun sound complete and specific. It usually refers to the receipt from that purchase.

Using no article is sometimes possible in Portuguese, but in a sentence like this, o comprovante sounds the most natural.

No. In Portuguese, pasta usually means something like:

• folder
• file
• binder
• sometimes briefcase or document holder, depending on context

So na pasta azul means in the blue folder, not anything related to food.

This is a classic false friend for English speakers.

Yes. Portuguese is fairly flexible with this kind of time expression.

All of these are possible:

• Depois da compra, eu vou guardar o comprovante na pasta azul.
• Eu vou guardar o comprovante na pasta azul depois da compra.
• Vou guardar o comprovante na pasta azul depois da compra.

Putting Depois da compra at the beginning helps set the time frame first. It sounds very natural and often slightly more organized or narrative.

The comma is also normal when that time phrase comes at the beginning.

A few very natural versions are:

• Depois da compra, vou guardar o comprovante na pasta azul.
• Depois da compra, eu vou guardar o comprovante na pasta azul.

These are both good everyday Brazilian Portuguese.

Depending on context, a Brazilian speaker might also say recibo instead of comprovante, but that changes the nuance a little:

• comprovante = proof/receipt, often broader
• recibo = receipt, often more specifically a receipt document

So your original sentence is already very natural.