Eu compro um abacate e um ananás no mercado.

Not usually. In Portuguese, the verb ending often makes the subject clear, so Compro um abacate e um ananás no mercado is already complete.

Here, compro ends in -o, which tells you it means I buy.

Using Eu is still correct. It can add:

• emphasis: Eu compro, não tu
• contrast
• clarity, if needed in context

So Eu is optional here, not wrong.

Compro is the first person singular present indicative of the verb comprar.

So:

• comprar = to buy
• compro = I buy

The ending -o is the normal ending for many regular -ar verbs in the I form:

• falo = I speak
• trabalho = I work
• compro = I buy

Yes, depending on context.

In Portuguese, the simple present can often express:

• a habit: I buy
• something happening now, especially in simple examples or conversation
• sometimes a near future meaning, depending on context

If you want to be very explicit about I am buying, Portuguese can also use:

• Estou a comprar in Portugal

But compro by itself is completely normal.

Because Portuguese normally uses an article before each singular countable noun here.

So:

• um abacate e um ananás = one avocado and one pineapple

Repeating um makes it clear that you mean one of each.
Leaving out the second um would sound unnatural in this sentence.

This is especially important because both abacate and ananás are singular count nouns.

Because both nouns are grammatically masculine:

• o abacate
• o ananás

So the indefinite article must also be masculine:

• um abacate
• um ananás

This is grammatical gender, not biological gender. Fruit names simply belong to a noun class, and the article has to match that class.

No is a contraction of:

• em
  • o = no

So:

• em o mercado becomes no mercado

This is extremely common in Portuguese. Other similar contractions are:

• na = em + a
• nos = em + os
• nas = em + as

So no mercado literally comes from in the market / at the market.

Because Portuguese very often uses the definite article with places after a preposition.

So expressions like these are very normal:

• no mercado
• no banco
• na escola
• no supermercado

English sometimes uses an article too, but Portuguese uses it very regularly in these everyday location expressions.

So em mercado would not sound natural here.

Yes. Ananás is the usual word for pineapple in Portugal.

That is one of the clearest vocabulary differences between Portugal and Brazil:

• Portugal: ananás
• Brazil: usually abacaxi

So this sentence sounds clearly European Portuguese.

No major difference here. Abacate is the standard word for avocado in both Portugal and Brazil.

So the regional vocabulary point in this sentence is really ananás, not abacate.

The stress falls on the last syllable:

• a-na-NÁS

The acute accent in ananás shows that the word is stressed there.

In European Portuguese, the final s is pronounced like sh at the end of a word, so it sounds roughly like:

• ah-nah-NASH

Without the accent, a learner might expect the stress somewhere else, so the written accent is helpful.

The stress is on the second-to-last syllable in both words:

• a-ba-CA-te
• mer-CA-do

That is the default stress pattern for many Portuguese words ending in a vowel, so they do not need a written accent mark.

That is why you see:

• abacate
• mercado

but:

• ananás with an accent, because its stress is not in the default position

In European Portuguese, the conjunction e is usually pronounced like ee.

So:

• abacate e um ananás

sounds roughly like:

• abacatee um ananás

It is not pronounced like English ay.

This is a very common pronunciation point for English speakers.

They are related, but not identical:

• mercado = market, often a general market or market building
• supermercado = supermarket
• feira = open-air market or street market

So no mercado is natural, but it does not specifically mean a supermarket.

Depending on the real situation, a Portuguese speaker might also say:

• no supermercado
• na feira

Yes, absolutely.

That version is very natural and probably more common in everyday Portuguese, because the subject pronoun is often omitted when it is already clear from the verb.

So both are correct:

• Eu compro um abacate e um ananás no mercado
• Compro um abacate e um ananás no mercado

The version with Eu is just slightly more explicit or emphatic.