Minha irmã tem uma namorada tranquila, e elas gostam de caminhar no parque.

Because irmã is a feminine noun in Portuguese, so the possessive has to agree with it:

• meu irmão = my brother
• minha irmã = my sister

In Portuguese, possessives like meu/minha, seu/sua, nosso/nossa usually agree with the thing possessed, not with the owner.

The ã shows a nasal sound, which is very important in Portuguese pronunciation.
So irmã is pronounced with a nasal ending, not like a plain a.

Also, irmã means sister, while the masculine form is:

• irmão = brother

Notice both use nasal vowels:

• ã in irmã
• ão in irmão

Tem is the 3rd person singular form of the verb ter (to have):

• eu tenho = I have
• você/ele/ela tem = you/he/she has
• minha irmã tem = my sister has

Since minha irmã is singular, tem is the correct form.

Usually, yes. In this sentence, namorada most naturally means girlfriend in the romantic sense.

• namorado = boyfriend
• namorada = girlfriend

Because the sentence later says elas (they, feminine), it strongly suggests that your sister and her girlfriend are both female.

In some contexts, learners wonder whether namorada could just mean female friend, but in normal Brazilian Portuguese it usually means a romantic partner, not just a friend.

Because adjectives usually agree in gender and number with the noun they describe.

Here, namorada is:

• feminine
• singular

So the adjective must also be feminine singular:

• tranquilo = calm, easygoing, relaxed (masculine singular)
• tranquila = calm, easygoing, relaxed (feminine singular)

Examples:

• um namorado tranquilo
• uma namorada tranquila

Here, tranquila means something like:

• calm
• easygoing
• relaxed
• laid-back

When describing a person, it often refers to personality or manner, not just temporary silence or stillness.

So uma namorada tranquila suggests a calm/easygoing girlfriend.

Portuguese has gendered plural pronouns:

• eles = they (all masculine, or mixed group)
• elas = they (all feminine)

Since the group is my sister + her girlfriend, both are female, so elas is correct.

Because the verb gostar normally requires the preposition de in Portuguese.

So you say:

• gostar de algo = to like something
• gostar de fazer algo = to like doing something

Examples:

• Eu gosto de música.
• Ela gosta de ler.
• Elas gostam de caminhar.

This is different from English, where like does not need a preposition.

Because the subject is elas (they), which is plural.

The verb gostar is conjugated like this in the present tense:

• eu gosto
• você/ele/ela gosta
• nós gostamos
• vocês/eles/elas gostam

So:

• ela gosta = she likes
• elas gostam = they like

After gostar de, the next verb stays in the infinitive:

• gostar de caminhar = to like to walk / to like walking
• gostar de dançar = to like dancing
• gostar de estudar = to like studying

So caminhar is not conjugated here because it follows another verb construction.

Both can relate to walking, but they are not always identical.

• caminhar often means to walk, especially as an activity, exercise, or stroll
• andar can mean to walk, but also to go around, to move, or even to ride/travel depending on context

In this sentence, caminhar no parque sounds very natural for going for walks in the park.

Because em + o contracts to no in Portuguese.

So:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

That means:

• no parque = in the park / at the park

This kind of contraction is extremely common in Portuguese.

Portuguese uses definite articles very frequently, often more than English learners expect.

Here, no parque is the normal way to say in the park.
Using the article sounds natural because Portuguese commonly says:

• na escola = at school
• no mercado = at the market
• no parque = in the park

So the article is not unusual here; it is the standard phrasing.

It is acceptable, but in a short sentence like this, many writers would also omit it:

• Minha irmã tem uma namorada tranquila e elas gostam de caminhar no parque.

The comma can help separate the two clauses:

1. Minha irmã tem uma namorada tranquila
2. elas gostam de caminhar no parque

So it is not strange, but it is not always required in simple writing.

Yes, if the context is already clear.

You could say:

• Ela tem uma namorada tranquila, e elas gostam de caminhar no parque.

But minha irmã is useful when you are first introducing who the person is. After that, Portuguese often switches to pronouns like ela.

In standard Portuguese, that would usually sound incomplete or unnatural here. Portuguese often allows omitted subjects, but the verb ending has to make the subject clear enough, and the omitted subject should be easy to recover.

While gostam does indicate a plural they, keeping elas is clearer and more natural in this sentence, especially for learners.

So:

• ... e elas gostam de caminhar no parque. = clear and natural

The subject elas shows that the action belongs to both of them:

• elas gostam = they like

So the sentence is not saying only your sister likes it or only the girlfriend likes it. It says the two women like walking in the park.