Si tuviera una duda sobre el contrato, preferiría preguntarle a una abogada antes de firmar nada.

Because this sentence is expressing a hypothetical or unlikely situation:

Si tuviera una duda... = If I had a question/doubt...

In Spanish, a common pattern for hypothetical situations is:

si + imperfect subjunctive + conditional

So here:

• si tuviera = if I had
• preferiría = I would prefer

Why not the others?

• si tenía would sound like a real past/imperfect situation: if I had / used to have
• si tendría is not standard here, because Spanish normally does not use the conditional after si in this kind of clause

So si tuviera... preferiría... is the expected structure.

It matches the hypothetical si clause.

The full pattern is:

• Si tuviera una duda...
• ...preferiría preguntarle...

This means:

• If I had a doubt/question...
• ...I would prefer to ask...

The conditional shows what the speaker would do in that hypothetical situation.

Compare:

• Si tengo una duda, prefiero preguntar... = If I have a question, I prefer to ask...
• Si tuviera una duda, preferiría preguntar... = If I had a question, I would prefer to ask...

The second one is more tentative and hypothetical.

Literally, duda means doubt, but in many contexts it is often better translated as:

• question
• uncertainty
• reservation
• something I’m not sure about

In this sentence, si tuviera una duda sobre el contrato is very natural Spanish, and in English you would often say:

• If I had a question about the contract...
• If I was unsure about something in the contract...

So yes, duda literally means doubt, but the most natural English translation here is often question or concern.

Because sobre here means about or regarding.

So:

• una duda sobre el contrato = a question/doubt about the contract

You could sometimes hear del contrato in other contexts, but sobre is especially clear and natural when talking about the topic of a question, concern, or discussion.

Examples:

• Tengo una pregunta sobre este tema. = I have a question about this topic.
• Hay dudas sobre el acuerdo. = There are doubts about the agreement.

So sobre is used because it introduces the subject matter.

Preguntarle = to ask him/her/you (formal)

The ending -le is an indirect object pronoun. It refers to the person being asked.

In this sentence:

preguntarle a una abogada

means:

• to ask a lawyer
• more literally, to ask her, a lawyer

Spanish often uses both:

1. the pronoun: le
2. the full phrase: a una abogada

This is very common and often sounds more natural than leaving the pronoun out.

So the structure is:

• preguntar algo a alguien = to ask something to someone / to ask someone something
• preguntarle a una abogada = to ask a female lawyer

It may seem repetitive from an English point of view, but in Spanish this is very normal.

Spanish often duplicates the indirect object:

• Le voy a escribir a mi hermano.
• Quiero preguntarle a una abogada.

The pronoun le helps mark the role of the person involved, and the phrase a una abogada specifies who that person is.

In English, we usually just say:

• I would prefer to ask a lawyer

But in Spanish, preguntarle a una abogada is completely natural.

Because the verb preguntar normally takes the pattern:

preguntar algo a alguien = to ask something to someone

Even when the something is not stated, the person being asked still appears with a:

• preguntarle a una abogada
• preguntar a un profesor
• preguntarle a María

This is not exactly the same as the personal a used with direct objects; here it is part of how preguntar connects to the person receiving the question.

Because abogada is the feminine form of lawyer.

• abogado = male lawyer
• abogada = female lawyer

The sentence specifically says a una abogada, so the speaker is referring to a female lawyer.

This could be because:

• the speaker has a female lawyer in mind
• the example simply chose a feminine noun
• the gender matters in context, or it may not matter much at all

If you wanted to make it masculine, you would say:

...preferiría preguntarle a un abogado...

In Spanish, nada can mean anything in certain contexts, especially after words like antes de, sin, or in questions and conditional-type ideas.

So:

antes de firmar nada = before signing anything

This is completely natural Spanish.

Why not algo?

• firmar algo = sign something
• firmar nada here means sign anything at all, which is stronger and more natural in this context

Why not no firmar nada?

Because that would mean not sign anything, which changes the meaning. The sentence is not saying before not signing anything. It is saying before signing anything.

Because in Spanish, when antes de is followed by a verb and there is no new subject introduced, the verb usually goes in the infinitive:

• antes de firmar = before signing
• antes de salir = before leaving
• antes de comer = before eating

So:

antes de firmar nada = before signing anything

If Spanish wants to introduce a different subject with a full clause, it may use another structure, for example:

• antes de que firmes nada = before you sign anything

But in your sentence, the infinitive is the normal and simple choice.

Yes, but the meaning changes.

• Si tengo una duda sobre el contrato, prefiero preguntarle a una abogada... = If I have a question about the contract, I prefer to ask a lawyer...

  • This sounds like a real or likely situation, or a general habit/preference.

• Si tuviera una duda sobre el contrato, preferiría preguntarle a una abogada... = If I had a question about the contract, I would prefer to ask a lawyer...

  • This sounds hypothetical, more tentative, or more polite.

• Si tenía una duda...

  • This is much less likely to be the intended choice here. It sounds like a past background situation and would need a different context.

So the original sentence uses the hypothetical pattern on purpose.

No, not in this sentence.

A true Spanish double negative would be something like:

• No firmé nada. = I didn’t sign anything.
• No vi a nadie. = I didn’t see anyone.

In antes de firmar nada, there is no negative word before the verb. Here nada works like anything rather than nothing.

So:

• No firmé nada = I signed nothing / I didn’t sign anything
• Antes de firmar nada = before signing anything

Same word, but different effect depending on the structure.

It is a common hypothetical conditional pattern in Spanish:

Si + imperfect subjunctive, conditional

In your sentence:

• Si tuviera una duda sobre el contrato,
• preferiría preguntarle a una abogada antes de firmar nada.

This pattern is used for:

• imaginary situations
• unlikely situations
• polite distancing
• advice or preferences stated less directly

More examples:

• Si fuera tú, lo leería otra vez. = If I were you, I would read it again.
• Si tuviera tiempo, iría contigo. = If I had time, I would go with you.
• Si supiera la respuesta, te la diría. = If I knew the answer, I would tell you.

So this sentence is a very useful model to learn.