Eu prefiro a sopa sem pimenta.

You can absolutely drop eu.

In Brazilian Portuguese, subject pronouns are often omitted when the verb already makes the subject clear. So:

• Eu prefiro a sopa sem pimenta.
• Prefiro a sopa sem pimenta.

Both are natural.

Including eu can add:

• emphasis: I prefer the soup without pepper.
• contrast: I prefer it, but someone else may not.

Because prefiro is the conjugated form of the verb preferir for I in the present tense.

• preferir = to prefer
• eu prefiro = I prefer

A few present-tense forms are:

• eu prefiro
• você prefere
• ele/ela prefere
• nós preferimos
• vocês preferem
• eles/elas preferem

So prefiro matches the subject eu.

Because sopa is a feminine singular noun, and a is the feminine singular definite article.

• a sopa = the soup

Portuguese often uses articles in places where English might not. With food and everyday nouns, using the article is very common and natural.

So a sopa can sound more complete and idiomatic than just sopa, depending on the context.

Yes, you could, but it changes the feel slightly.

• Eu prefiro a sopa sem pimenta sounds more natural in many everyday situations.
• Eu prefiro sopa sem pimenta sounds a bit more general, broad, or menu-like.

In Portuguese, articles are used more often than in English, so learners often leave them out too much.

Sem means without.

It is a preposition, and it does not change form. It stays sem no matter what comes after it.

Examples:

• sem açúcar = without sugar
• sem sal = without salt
• sem pimenta = without pepper

So sem is very straightforward.

Because pimenta is being used in a general sense here: without pepper.

After sem, Portuguese often uses a noun without an article when talking about something in a general or uncountable way.

So:

• sem pimenta = without pepper

If you said sem a pimenta, that would usually sound like you mean a specific pepper or a specific serving of pepper, not pepper in general.

In Brazilian Portuguese, pimenta often suggests chili pepper or spicy pepper, depending on context.

If you specifically mean black pepper, Brazilians often say:

• pimenta-do-reino

So sem pimenta in this sentence will often be understood as:

• without chili pepper
• not spicy
• without hot pepper

Context matters, but that is a very common interpretation in Brazil.

Here, sem pimenta describes a sopa, so it naturally comes right after sopa:

• a sopa sem pimenta = the soup without pepper

That is the most natural word order.

You generally would not move it to the beginning unless you were creating a special emphasis or a different structure. For a learner, the safest pattern is:

• noun + sem + noun

Examples:

• café sem açúcar
• salada sem molho
• sopa sem pimenta

Yes.

A common structure is:

• preferir X a Y

For example:

• Prefiro a sopa sem pimenta à sopa com pimenta.

Notice the à there:

• a + a = à

That happens because:

• preferir ... a ... and
• a sopa already begins with a

So:

• a
  • a sopa → à sopa

In everyday Brazilian Portuguese, you may also hear:

• Prefiro a sopa sem pimenta do que a sopa com pimenta.

That is very common in speech, even though some grammar books prefer a.

A rough pronunciation guide is:

eh-ooh preh-FEE-roo ah SOH-pah seng pee-MEN-tah

A few helpful notes:

• prefiro: the stress is on fi
• the final o in prefiro often sounds like u
• sem has a nasal sound; it is not a plain English sem
• pimenta has the stress on men

This is only an approximation, but it is enough to help you say it more naturally.