Minha irmã prefere suco de laranja, mas meu irmão quer levar uma banana para o trabalho.

Because minha and meu agree with the thing being possessed, not with the person who owns it.

• irmã is feminine, so you use minha
• irmão is masculine, so you use meu

So:

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

This same pattern works with other possessives too:

• meu livro = my book
• minha casa = my house

Prefere is the third person singular of the verb preferir in the present tense.

Here it matches minha irmã, which is equivalent to ela:

• ela prefere = she prefers

A quick mini-pattern:

• eu prefiro = I prefer
• você / ele / ela prefere = you / he / she prefers
• nós preferimos = we prefer
• eles / elas preferem = they prefer

So Minha irmã prefere... literally follows the same pattern as Ela prefere...

Portuguese usually uses de to show what something is made from, flavored with, or associated with.

So:

• suco de laranja = orange juice
• literally: juice of orange

This structure is very common:

• suco de maçã = apple juice
• bolo de chocolate = chocolate cake
• camisa de algodão = cotton shirt

English often puts one noun in front of another, but Portuguese usually links them with de.

In Portuguese, food and drink items often appear without an article when speaking generally.

So prefere suco de laranja sounds natural for prefers orange juice.

You could also hear:

• prefere o suco de laranja if you mean a specific orange juice, or orange juice as a category in contrast to something else

For example:

• Ela prefere suco de laranja. = She prefers orange juice.
• Ela prefere o suco de laranja ao de uva. = She prefers the orange juice to the grape juice / orange juice over grape juice.

Both are possible, but the version without the article is very natural in a general statement.

After quer (from querer, meaning to want), Portuguese normally uses another verb in the infinitive.

So:

• quer levar = wants to take
• literally: wants take

This is the same pattern as in English:

• She wants to eat
• Ele quer comer

More examples:

• quero dormir = I want to sleep
• ela quer estudar = she wants to study
• meu irmão quer levar uma banana = my brother wants to take a banana

This is a very common question.

• levar = to take, to carry away from where the speaker is
• trazer = to bring toward where the speaker is

In this sentence, levar is used because the brother is taking the banana to work, away from his current location.

Compare:

• Vou levar uma banana para o trabalho. = I’m going to take a banana to work.
• Vou trazer uma banana para o trabalho. = I’m going to bring a banana to work.

The second one could make sense depending on where the speaker is and what viewpoint is being used, but levar is the most natural default here.

Uma banana means a banana. The word uma is the feminine singular indefinite article.

• um = a / an for masculine nouns
• uma = a / an for feminine nouns

Since banana is feminine, you say uma banana.

Compare:

• um livro = a book
• uma banana = a banana

Portuguese often uses indefinite articles in places where English also uses them, especially when talking about one countable item.

Para o means to the or for the, depending on context.

Here:

• para = to / for
• o trabalho = work / the workplace
• para o trabalho = to work / for work

In natural English, we say to work, but Portuguese often says para o trabalho, which is more literally to the workplace or for work.

Also, in everyday Brazilian Portuguese, para o is often contracted in speech and writing to:

• pro

So you will often hear:

• Meu irmão quer levar uma banana pro trabalho.

That is very common and natural in Brazil.

Sometimes yes, but the meaning can shift a little.

• ir ao trabalho often means to go to work
• levar algo para o trabalho means to take something to work

With levar, para o trabalho is the most natural choice because it shows destination or purpose clearly.

So:

• levar uma banana para o trabalho = take a banana to work

Using ao trabalho here would sound less natural in everyday speech.

Mas means but.

It connects two contrasting ideas:

• Minha irmã prefere suco de laranja
• mas meu irmão quer levar uma banana para o trabalho

So the sentence contrasts the sister’s preference with the brother’s intention.

It is the standard Portuguese word for but in this kind of sentence.

Because mas joins two full clauses, and Portuguese normally uses a comma before it in that situation.

Here the two clauses are:

1. Minha irmã prefere suco de laranja
2. mas meu irmão quer levar uma banana para o trabalho

This is very similar to English punctuation with but.

The ã in irmã shows a nasal sound, which is very important in Portuguese pronunciation.

So irmã is not pronounced like a plain a at the end. The vowel is nasalized.

Also, the accent mark helps show the correct written form of the word. Portuguese spelling uses accents to mark pronunciation patterns and stress.

A useful pair:

• irmã = sister
• irmão = brother

Both contain nasal vowels:

• ã in irmã
• ão in irmão

They are the masculine and feminine forms of the noun:

• irmão = brother
• irmã = sister

This is a common gender pair in Portuguese, but it is a little irregular because the endings are not just -o and -a in the usual way.

Related forms:

• meu irmão = my brother
• minha irmã = my sister
• irmãos = brothers / siblings / brothers and sisters, depending on context
• irmãs = sisters

Yes, but the meaning is slightly different.

• prefere suco de laranja = prefers orange juice
• gosta mais de suco de laranja = likes orange juice more

Preferir directly expresses preference.
Gostar mais de expresses liking something more than something else.

So prefere is the cleaner and more direct choice here.

A little, but the given order is the most natural.

Standard order here is:

subject + verb + object

• Minha irmã
  • prefere
    • suco de laranja
• meu irmão
  • quer levar
    • uma banana para o trabalho

You could rearrange parts for emphasis, but it may sound marked or less neutral. For learners, the given word order is the best pattern to follow.

Yes, it is natural and correct.

In everyday speech, though, some parts might sound a little more conversational:

• Minha irmã prefere suco de laranja, mas meu irmão quer levar uma banana pro trabalho.

The main difference is:

• para o → pro

That contraction is extremely common in spoken Brazilian Portuguese. The original sentence is still perfectly normal, especially in careful speech or writing.