¿Podría usted decirme si la merluza lleva perejil o si viene con vinagreta?

Podría is the conditional form of poder, and here it makes the request more polite.

• ¿Puede usted decirme...? = Can you tell me...?
• ¿Podría usted decirme...? = Could you tell me...?

In restaurant or customer-service situations in Spain, podría sounds courteous and natural.

Yes, it can be left out.

Spanish often omits subject pronouns because the verb form already shows who the subject is. So both of these are correct:

• ¿Podría usted decirme...?
• ¿Podría decirme...?

Including usted adds formality or emphasis. In this sentence, it helps make the tone especially polite.

Because object pronouns like me have fixed positions in Spanish.

With a conjugated verb + infinitive, you usually have two correct options:

• attach the pronoun to the infinitive: podría decirme
• put the pronoun before the conjugated verb: me podría decir

So these are both correct:

• ¿Podría decirme si...?
• ¿Me podría decir si...?

But me decir by itself is not the normal structure.

Here si means whether or if in an indirect yes/no question.

It does not mean a condition like if it rains.

So:

• decirme si la merluza lleva perejil = tell me whether the hake has parsley

This is very common after verbs like:

• saber = to know
• preguntar = to ask
• decir = to tell

For example:

• No sé si está abierto. = I don’t know whether it’s open.

The second si is there to keep the two alternatives parallel and clear.

• si la merluza lleva perejil o si viene con vinagreta

This is fully correct and natural.

You will also hear:

• si la merluza lleva perejil o viene con vinagreta

That is also correct. Repeating si just makes the structure a bit clearer, especially in careful or polite speech.

In food Spanish, especially in Spain, llevar is very commonly used to mean to contain, to include, or to have as an ingredient.

So:

• La merluza lleva perejil. = The hake contains parsley / has parsley in it.

This is a very useful restaurant word. Other examples:

• ¿Lleva ajo? = Does it have garlic?
• ¿Este plato lleva nueces? = Does this dish contain walnuts?

It does not literally mean the fish is carrying parsley; it is just the normal culinary use.

They are similar, but there is a small nuance.

• lleva perejil usually suggests parsley is part of the preparation or ingredients
• viene con vinagreta usually suggests the dish is served with vinaigrette

So:

• llevar = what the dish contains
• venir con = what the dish comes with

In real life, the difference is sometimes not very strict, but this is the general idea.

Because Spanish often uses the definite article with foods, dishes, and menu items.

• la merluza = the hake

In a restaurant context, this usually refers to the hake dish being served or offered.

Also, merluza is a feminine noun, so it takes la.

Because when Spanish talks about ingredients or accompaniments in a general way, it often uses the noun without an article.

• lleva perejil
• viene con vinagreta

That sounds natural when you mean parsley or vinaigrette as substances or ingredients in general.

If you add an article, the meaning often becomes more specific:

• con la vinagreta de la casa = with the house vinaigrette
• con una vinagreta suave = with a mild vinaigrette

So the version in your sentence is the normal general-use pattern.

Yes, it is fairly formal and polite because of podría and usted.

A less formal version in Spain would be:

• ¿Podrías decirme si la merluza lleva perejil o si viene con vinagreta?

Or even more simply:

• ¿La merluza lleva perejil o viene con vinagreta?

That last version is direct but still perfectly normal in a casual restaurant situation.

Yes, absolutely.

These are both correct:

• ¿Podría usted decirme si...?
• ¿Me podría decir si...?

The second one is often very natural in everyday speech. The meaning is the same; the difference is just word order and tone.

Because Spanish does not require the same kind of subject-auxiliary inversion that English does.

In English, you say:

• Could you tell me...?

You have to put could before you.

In Spanish, questions are often shown mainly by:

• intonation in speech
• the question marks ¿...? in writing

So Spanish can keep a normal word order:

• ¿Podría usted decirme...?

No special inversion is necessary.