Eu prefiro morangos com iogurte, mas o meu irmão gosta mais de pêssego e laranja.

That’s true: Portuguese often leaves out the subject pronoun because the verb ending already shows who is doing the action.

So Prefiro morangos com iogurte is perfectly natural.

Here, Eu is included for clarity or contrast. The sentence sets up a contrast between I and my brother:

Eu prefiro..., mas o meu irmão...

So the Eu helps highlight that contrast, even though it is not strictly necessary.

Prefiro is the 1st person singular present form of preferir, meaning to prefer.

So:

• eu prefiro = I prefer
• tu preferes = you prefer
• ele/ela prefere = he/she prefers

A useful point: preferir already contains the idea of liking one thing more than another, so in standard Portuguese you normally do not add mais to it.

Natural:

• Prefiro morangos.
• Prefiro morangos a bananas.

Not the usual standard phrasing:

• Prefiro mais morangos.

Because com means with, and here it describes a combination of foods:

• morangos com iogurte = strawberries with yogurt

That means the strawberries are being eaten together with yogurt.

If you used de, the meaning would usually change. In food contexts, de often means made of, containing, or flavoured with:

• iogurte de morango = strawberry yogurt
• sumo de laranja = orange juice

So com is the right choice for strawberries with yogurt.

In European Portuguese, it is very common to use the definite article before a possessive:

• o meu irmão = my brother
• a minha irmã = my sister
• os meus amigos = my friends

So in Portugal, o meu irmão is the most standard and natural form.

Leaving out the article is more common in Brazilian Portuguese, or in certain special expressions.

Because the possessive agrees with the thing possessed, not with the speaker.

The noun irmão is masculine singular, so you use:

• o meu irmão

If the noun were feminine, you would use:

• a minha irmã

So the pattern is:

• meu for masculine singular nouns
• minha for feminine singular nouns
• meus / minhas for plurals

Because gostar normally requires the preposition de.

So the pattern is:

• gostar de + noun
• gostar de + infinitive

Examples:

• Gosto de morangos.
• Ela gosta de café.
• Gostamos de viajar.

That means gosta mais de pêssego e laranja follows the normal structure of the verb.

If there is an article after de, it usually contracts:

• de + o = do
• de + a = da

For example:

• gosta do iogurte
• gosta da laranja

Gosta mais de literally means likes ... more.

In many contexts, especially everyday speech, it is very close to prefers.

So:

• O meu irmão gosta mais de pêssego e laranja
  can be understood as
• My brother prefers peach and orange or
• My brother likes peach and orange more

You could also say:

• O meu irmão prefere pêssego e laranja

The difference is mostly one of style and nuance:

• preferir = more direct, more exact
• gostar mais de = often a bit softer and more conversational

Because mais is modifying the idea of liking, not the noun itself.

So:

• gosta mais de X = likes X more

The usual order is:

gostar + mais + de + noun

Examples:

• Gosto mais de café.
• Ela gosta mais de verão.

If you want to make the comparison explicit, you can add:

• do que or repeat de

For example:

• Ele gosta mais de laranja do que de maçã.

Portuguese often talks about foods in a general way, and the singular/plural choice can depend on how the speaker is thinking about them.

Here:

• morangos com iogurte sounds like an actual serving or dish: strawberries with yogurt
• pêssego e laranja sounds more like fruit types or flavours/categories in general

That is why the singular can sound natural.

Also, after gostar de, Portuguese often uses nouns without an article when talking about things in general:

• gosto de café
• gosto de chocolate
• gosto de laranja

So de pêssego e laranja is a normal general statement.

You could also hear:

• de pêssegos e laranjas

That would sound a bit more clearly like peaches and oranges as countable fruits.

They give you pronunciation information.

In pêssego:

• ê shows a stressed closed e sound

In irmão:

• ã shows a nasal vowel
• the ending -ão is one of the most common nasal endings in Portuguese

So these accents are not optional spelling marks: they help tell you how the word is pronounced.

They also help show stress:

• pêssego → stress on the first syllable
• irmão → stress on the last syllable

For an English speaker, ão is especially important to practise, because it does not sound like a normal English vowel + n combination.