Learn Hardcore Spanish (Latin America): ¿A quién le pregunto si tengo una duda sobre la lección? - Who do I ask if I have a doubt about the lesson?

quién alone is typically the subject (Who is coming? → ¿Quién viene?).
a quién is used when that person is an object of the verb (Whom do I ask? → ¿A quién pregunto?).

Here, the person you ask is the indirect object of preguntar, so Spanish needs a → a quién.

What is the le doing in ¿A quién le pregunto…?? Why is it needed?

Le is an indirect object pronoun meaning to him / to her / to you (formal) / to that person.

In Spanish, it’s very common (and often natural-sounding) to “double” the indirect object:

Le pregunto a la maestra. = I ask the teacher.
¿A quién le pregunto? = To whom do I ask? / Who do I ask?

Here:

a quién = explicitly names the person (but as a question).
le = refers back to that person grammatically.

This “doubling” of le plus a + person is normal and very frequent in spoken Spanish. It’s not a mistake or unnecessary redundancy.

Can I drop le and just say ¿A quién pregunto…??

You can say ¿A quién pregunto? and people will understand you. It is grammatically possible.

However, in everyday Latin American Spanish, ¿A quién le pregunto…? sounds more natural and more typical, because:

preguntar “likes” to take an indirect object pronoun when referring to a person you’re asking.
The le makes the sentence feel more complete and idiomatic.

So:

Correct but less common: ¿A quién pregunto si tengo una duda…?
More natural: ¿A quién le pregunto si tengo una duda…?

If we already have a quién, why do we still need le? Isn’t that double?

Yes, it is technically doubled, but Spanish allows and prefers this with indirect objects. This is called clitic doubling.

Pattern:

Le pregunto a Juan.
Le dije a mi mamá.
Le escribí a tu profesor.

The indirect object pronoun (le) keeps the grammar clear and attaches to the verb; a Juan / a mi mamá / a tu profesor specifies who that person is.

In questions with a quién, we keep the same pattern:

¿A quién le pregunto?
¿A quién le escribo?
¿A quién le digo esto?

So it’s not a mistake; it’s a very standard structure.

What does pregunto mean exactly, and why is it in that form?

Pregunto is the first-person singular, present tense of preguntar (to ask).

yo pregunto = I ask / I am asking

Spanish usually omits the subject pronoun yo when it’s obvious from the verb form:

(Yo) pregunto. = I ask.
In the question: ¿A quién le pregunto…? = Who do I ask…?

So pregunto here means “I ask” or “I should ask / am supposed to ask”, depending on context, but grammatically it’s just present indicative.

Could I say ¿Le pregunto a quién…? instead of ¿A quién le pregunto…??

You can say ¿Le pregunto a quién? but it usually has a different feel:

¿Le pregunto a quién? often sounds like an echo question or a reaction:
Someone told you: “Le preguntas a Marta.”
You reply: ¿Le pregunto a quién? → I’m supposed to ask *who?*

For a normal information question (open question) asking “Who should I ask?”, Spanish strongly prefers to place the question word at the beginning:

¿A quién le pregunto si tengo una duda sobre la lección?

So the original order is the neutral, standard way to ask.

Why is it si tengo and not si tenga? I thought si often took the subjunctive.

With si meaning if, Spanish normally uses the indicative when the condition is real or possible:

Si tengo una duda, le pregunto al profesor.
= If I have a question, I ask the teacher.

The subjunctive after si appears mainly in unreal or very hypothetical conditions, and usually in the past forms:

Si tuviera / tuviese una duda, le preguntaría al profesor.
= If I had a question (hypothetically), I would ask the teacher.

In your sentence, the speaker is talking about a real, practical situation (I might really have a doubt during the lesson), so indicative is correct:

si tengo una duda = if I (do) have a doubt

What is the difference between tengo una duda and tengo una pregunta?

Both are common in classrooms, but the nuance is slightly different:

una duda = a doubt / something you’re unsure about
- Tengo una duda sobre la lección.
  → I’m not clear about part of the lesson; I’m uncertain and want clarification.
una pregunta = a question (a thing you ask)
- Tengo una pregunta sobre la lección.
  → I have a specific question I want to ask.

In real life, especially in Latin American Spanish:

Students very often say tengo una duda to mean “I don’t fully understand; can you explain?”
tengo una pregunta is also correct and common.

Your sentence with duda is very natural in a learning context.

What exactly does sobre la lección mean, and could I use de la lección instead?

Sobre here means about / regarding / on the topic of.

una duda sobre la lección
= a doubt about the lesson

You can also say:

una duda de la lección
una pregunta de la lección

Both are understood and used, but:

sobre feels a bit more specific: regarding the content of the lesson.
de is more general: of / from the lesson, also fine.

In many situations, sobre la lección and de la lección are practically interchangeable. The original sentence with sobre is very natural.

Does sobre la lección describe duda or pregunto?

In ¿A quién le pregunto si tengo una duda sobre la lección?, sobre la lección is most naturally understood as describing duda:

una duda sobre la lección = a doubt about the lesson

So the structure is:

si tengo [una duda sobre la lección]

It is not “To whom do I ask about the lesson, if I have a doubt?” but rather:

“If I have a doubt (which is about the lesson), whom should I ask?”

What gender is lección, and how do I know which article to use?

Lección is a feminine noun.

la lección = the lesson
una lección = a lesson

Clues:

Many words ending in -ción are feminine:
- la información, la explicación, la conversación, la situación, la lección

So you correctly say:

la lección, not el lección

Is this sentence formal or informal? Would it change with usted?

¿A quién le pregunto si tengo una duda sobre la lección? is neutral in terms of formality. It doesn’t explicitly show tú or usted, because:

The subject is yo (I), which is omitted.
There is no direct address to another person.

You can use this sentence when speaking to:

a teacher
a classmate
an administrator

If you wanted to make it a bit more formally phrased, you might use the conditional or add a modal verb, but it’s not necessary:

¿A quién debería preguntarle si tengo una duda sobre la lección?
¿A quién le podría preguntar si tengo una duda sobre la lección?

How does preguntar a alguien work compared to preguntar algo and preguntar por?

Preguntar has a few main patterns:

preguntar algo = to ask something
- Pregunto la hora. = I ask the time.
- Le pregunto la respuesta. = I ask him/her for the answer.
preguntar a alguien = to ask someone
- Le pregunto al profesor. = I ask the teacher.
- ¿A quién le pregunto? = Who do I ask?
preguntar por alguien = to ask about someone’s whereabouts or situation
- Preguntaron por ti. = They asked about you / They were asking for you.

In your sentence, it’s pattern (2): preguntar a alguien → ¿A quién le pregunto…? = Who should I ask?