No tengo dudas al respecto.

Because Spanish expresses having doubts with the verb tener: tener dudas.

So:

• Tengo dudas = I have doubts
• No tengo dudas = I have no doubts / I don’t have any doubts

Using ser or estar here would sound unnatural, because in Spanish you normally do not say I am doubts or I am doubtful in the same structure.

If you wanted a different wording, you could say:

• No dudo = I don’t doubt it
• Estoy seguro/a = I’m sure

In Spanish, tener dudas is very commonly used in the plural, even when English might say to have doubt in a more general sense.

So:

• Tengo dudas = I have doubts
• No tengo dudas = I have no doubts

The singular also exists:

• No tengo ninguna duda = I don’t have any doubt / I have no doubt

Both are correct, but no tengo dudas is a very natural general statement.

Al respecto means something like:

• about that
• regarding that
• on that matter
• in that respect (depending on context)

It is a fixed expression used to refer back to something already mentioned.

So No tengo dudas al respecto means:

• I have no doubts about that
• I have no doubts on that matter

It sounds a bit more formal or polished than simply saying sobre eso.

Because a + el contracts to al in Spanish.

So:

• a + el = al

That is why you say:

• al respecto

This contraction is required in almost all cases with a + el.

A similar contraction happens with:

• de + el = del

Yes. No tengo dudas sobre eso is also correct and easy to understand.

Comparison:

• No tengo dudas al respecto = a bit more formal, more fixed-expression-like
• No tengo dudas sobre eso = more direct and transparent for learners
• No tengo dudas acerca de eso = also correct, slightly formal

All of them can mean basically the same thing. In writing or formal speech, al respecto is especially common.

In Spanish, standard negation usually puts no before the conjugated verb.

So:

• Tengo dudas = I have doubts
• No tengo dudas = I don’t have doubts

This is the normal pattern:

• No sé = I don’t know
• No quiero = I don’t want
• No entiendo = I don’t understand

Yes, absolutely. That is a very common alternative.

Compare:

• No tengo dudas al respecto
• No tengo ninguna duda al respecto

Both mean essentially I have no doubts about that.

The version with ninguna duda can sound a little more emphatic, because ninguna reinforces the negative idea:

• ninguna = not any / none

Note that in Spanish, this is normal negative concord:

• No tengo ninguna duda Literally it looks like I don’t have no doubt, but in Spanish that is correct and standard.

It is possible in some contexts, but No tengo dudas or No tengo ninguna duda are much more natural.

Most speakers would usually prefer:

• No tengo dudas al respecto or
• No tengo ninguna duda al respecto

So for a learner, it is safest to use one of those two.

It is neutral to somewhat formal.

• No tengo dudas is completely normal in everyday Spanish.
• al respecto gives it a slightly more formal, polished, or written feel.

In casual conversation, someone might also say:

• No tengo dudas sobre eso
• No me cabe duda
• Lo tengo clarísimo (more informal/emphatic, depending on context)

So the sentence is very natural, but it sounds a bit more formal than the simplest everyday phrasing.

It sounds perfectly natural in Spain.

A speaker from Spain would understand it immediately, and it would fit especially well in:

• conversations with a serious tone
• interviews
• news-style language
• professional or academic contexts

In very casual speech, a person in Spain might more often say:

• No tengo ninguna duda
• No tengo dudas sobre eso
• Está clarísimo

But No tengo dudas al respecto is still fully natural.

A simple pronunciation guide:

• dudas ≈ DOO-das
• respecto ≈ res-PEK-to
• al respecto ≈ al res-PEK-to

A few useful notes for Spain:

• d in dudas is softer than English d between vowels.
• r in respecto is a tapped Spanish r, not a strong English r.
• The stress falls on:
  • DU in dudas
  • PEC in respecto

Full sentence:

• No tengo dudas al respecto
• roughly: no TEN-go DOO-das al res-PEK-to

Yes, if the context is already clear.

• No tengo dudas. = I have no doubts.
• No tengo dudas al respecto. = I have no doubts about that.

Adding al respecto makes the reference more explicit. Without it, the sentence is shorter and still natural if everyone already knows what that refers to.

Yes. Here are some natural alternatives:

• No tengo ninguna duda. = I have no doubt.
• No dudo de eso. = I don’t doubt that.
• No me cabe ninguna duda. = I have absolutely no doubt.
• Estoy seguro/a de eso. = I’m sure about that.

They are similar, but not always identical in tone:

• No tengo dudas al respecto sounds calm and firm.
• No me cabe ninguna duda sounds stronger and more emphatic.
• Estoy seguro/a focuses more on certainty than on the absence of doubts.