Eu comprei outra bolsa, mas ainda prefiro a antiga.

Yes, you often can leave it out.

Portuguese verbs usually show who the subject is, so comprei already means I bought, and prefiro already means I prefer. Because of that, eu is optional here.

• Eu comprei outra bolsa, mas ainda prefiro a antiga.
• Comprei outra bolsa, mas ainda prefiro a antiga.

Both are natural.

Including eu can add:

• emphasis
• clarity
• a slightly stronger personal tone

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

Comprei is the preterite tense, first-person singular, from the verb comprar (to buy).

So:

• eu comprei = I bought

The preterite is used for a completed action in the past. In this sentence, it means the speaker bought the bag and that action is finished.

Compare:

• Eu comprei outra bolsa. = I bought another bag.
• Eu comprava outra bolsa. = I used to buy / was buying another bag depending on context

So comprei is the straightforward past tense for a finished event.

Both are possible, but outra bolsa is the more basic and common choice here.

• outra bolsa = another bag / a different bag
• uma outra bolsa = also possible, but can sound a bit more emphatic or stylistically heavier

In everyday Portuguese, outro/outra often works without um/uma:

• Quero outro café. = I want another coffee.
• Ela comprou outra camisa. = She bought another shirt.

So outra bolsa is completely natural.

Here, outra most naturally means another or a different.

So:

• outra bolsa = another bag
• a antiga = the old one / the previous one

The sentence contrasts the newly bought bag with the older one.

A useful idea:

• outro/outra often means another when singular
• it can also mean other/different, depending on context

Here, the sense is: the speaker bought a different bag, but still prefers the old one.

Because Portuguese can use article + adjective to stand for a noun that is already understood.

So:

• a antiga literally means the old one
• the noun bolsa is omitted because it is already known from earlier in the sentence

This is very common in Portuguese.

Compare:

• Comprei uma camisa nova, mas prefiro a velha. = I bought a new shirt, but I prefer the old one.

• Ela viu duas casas e escolheu a maior. = She saw two houses and chose the bigger one.

So a antiga is a natural shorthand for a bolsa antiga.

Because they agree with bolsa, which is a feminine singular noun.

In Portuguese, adjectives and words like outro/outra usually agree in gender and number with the noun.

Here:

• bolsa = feminine singular
• so we get outra bolsa
• and later a antiga (referring back to bolsa)

Compare:

• outro livro = another book
• a antiga = the old one (feminine)
• o antigo = the old one (masculine)

So the feminine forms are used because bolsa is feminine.

Here, ainda means still.

So:

• ainda prefiro a antiga = I still prefer the old one

It shows that even after buying another bag, the speaker’s preference has not changed.

Common meanings of ainda include:

• still
• yet
• sometimes even

In this sentence, still is the right meaning.

Examples:

• Ainda estou cansado. = I’m still tired.
• Ela ainda não chegou. = She still hasn’t arrived / She hasn’t arrived yet.

Because that is a very natural position for adverbs like ainda in Portuguese.

• ainda prefiro a antiga = natural
• prefiro ainda a antiga = possible in some contexts, but less neutral here

Placing ainda before the verb clearly emphasizes the idea of still.

Portuguese word order with adverbs is flexible, but some positions sound more natural than others. In this sentence, mas ainda prefiro a antiga is the most standard and idiomatic wording.

Preferir means to prefer.

In this sentence:

• prefiro = I prefer

Here it takes a direct object:

• prefiro a antiga = I prefer the old one

If you want to state both things explicitly, Portuguese often uses this pattern:

• Prefiro X a Y. = I prefer X to Y.

For example:

• Prefiro café a chá. = I prefer coffee to tea.
• Prefiro a antiga à outra. = I prefer the old one to the other one.

In your sentence, the second item is already understood from context, so it does not need to be repeated.

Because mas means but and introduces a contrast between two clauses.

In standard written Portuguese, a comma is normally used before mas:

• Eu comprei outra bolsa, mas ainda prefiro a antiga.

This is similar to English, where a comma is also often used before but when joining two full clauses.

So the comma helps mark the contrast:

• first idea: I bought another bag
• second idea: I still prefer the old one

Yes, but it would mean something slightly different.

• outra bolsa = another / a different bag
• uma bolsa nova = a new bag

These ideas often overlap, but they are not identical.

Outra focuses on difference:

• it is another one, not the same one

Nova focuses on newness:

• it is new, not old

So:

• Comprei outra bolsa = I bought another/different bag
• Comprei uma bolsa nova = I bought a new bag

Your original sentence emphasizes contrast between one bag and the previous one, not necessarily the fact that the new one is brand-new.

A little, yes, but the original order is the most natural neutral version.

For example, these are possible:

• Comprei outra bolsa, mas ainda prefiro a antiga.
• Eu comprei outra bolsa, mas prefiro ainda a antiga.
• Outra bolsa eu comprei, mas ainda prefiro a antiga.
  This sounds more marked or emphatic.

The original sentence is the best standard version for most situations:

• Eu comprei outra bolsa, mas ainda prefiro a antiga.

It sounds natural, clear, and idiomatic in Brazilian Portuguese.