Learn Hardcore Portuguese (Portugal): Eu não tenho nenhuma dúvida agora.

In Portuguese, this is normal and correct. Portuguese loves “double negatives”; they don’t cancel each other out, they reinforce the negation.

Não = not
nenhuma (before a noun) = any / no (in negative contexts)

So:

Eu não tenho nenhuma dúvida.
Literally: I not have no/any doubt.
Natural English: I don’t have any questions / I have no doubts.

Other very common patterns in Portuguese:

Não vejo nada. – I don’t see anything / I see nothing.
Não conheço ninguém. – I don’t know anybody / I know nobody.

So: in Portuguese, não + negative word (nenhum, nada, ninguém, nunca…) is exactly how you normally say “not … any / no …” in English.

Could I leave out eu and just say Não tenho nenhuma dúvida agora?

Yes. In fact, that’s very natural in European Portuguese.

Portuguese is a “pro-drop” language: you can often drop the subject pronoun because the verb ending shows the person:

Tenho already shows 1st person singular (I).
So (Eu) não tenho nenhuma dúvida agora.
Both versions are correct.

When do people keep “eu”?

To contrast with someone else:
Eu não tenho nenhuma dúvida, mas ele tem.
I don’t have any questions, but he does.
To emphasise:
Eu não tenho nenhuma dúvida! – I have absolutely no doubt!

In a neutral context, Não tenho nenhuma dúvida agora is perfectly fine and common.

Why is it nenhuma dúvida (feminine) and not nenhum dúvida (masculine)?

Because “nenhum/nenhuma” must agree in gender and number with the noun it modifies.

dúvida is a feminine noun in Portuguese:
- a dúvida
- uma dúvida
- nenhuma dúvida

So:

Feminine singular: nenhuma dúvida
Feminine plural: nenhumas dúvidas (possible, but less common in everyday speech)
Masculine examples:
- nenhum problema (problem – masculine)
- nenhum livro (book – masculine)

Whenever you learn a new noun in Portuguese, it helps to learn it with the article:

a dúvida (feminine)
o problema (masculine)

What’s the difference between “nenhuma dúvida” and “dúvida nenhuma”?

Both are used, but there’s a difference in style and emphasis.

nenhuma dúvida (before the noun)
Neutral, very common:
- Eu não tenho nenhuma dúvida agora.
dúvida nenhuma (after the noun)
More emphatic, often sounds stronger or more informal:
- Eu não tenho dúvida nenhuma.
  (Very close to I don’t have any doubt at all / No doubt whatsoever.)

In actual use:

Talking about questions for the teacher:
Não tenho nenhuma dúvida agora. is more usual.
Emphasising certainty or strong denial:
Não tenho dúvida nenhuma de que isto está certo.
I have absolutely no doubt that this is right.

Does dúvida mean “doubt” or “question”? Why is it used when we mean “questions”?

Literally, dúvida means “doubt”, but in everyday Portuguese it often means “question” in the sense of something I’m not sure about / something I want you to explain.

So in a class or explanation context:

Tens alguma dúvida? – Do you have any questions?
Não tenho nenhuma dúvida. – I don’t have any questions.

If you translate it literally as doubt, it sounds odd in English (I don’t have any doubt now), but in Portuguese that’s the standard way to ask/answer:

Teacher: Alguma dúvida? – Any questions?
Student: Não, não tenho nenhuma dúvida. – No, I don’t have any questions.

Is there a difference between “Eu não tenho nenhuma dúvida agora” and “Eu não tenho dúvida agora” (without “nenhuma”)?

Yes, there’s a nuance difference:

Eu não tenho nenhuma dúvida agora.
→ Standard, natural way to say I don’t have any questions now.
Eu não tenho dúvida agora.
→ Grammatically possible, but sounds more like I don’t have (any) doubt now, i.e. I’m not in doubt / I’m sure now. In everyday speech it’s less common in this specific context.

To sound natural when talking about questions for the teacher, keep nenhuma:

(Eu) não tenho nenhuma dúvida agora.

Can I say “Eu não tenho quaisquer dúvidas agora” or “Eu não tenho qualquer dúvida agora”? Do they sound natural?

Yes, but they sound more formal and less colloquial than the original.

Eu não tenho qualquer dúvida agora.
– Correct, a bit formal / careful; often used in written language or polite speech.
Eu não tenho quaisquer dúvidas agora.
– Also correct, but sounds quite formal or legalistic (you might see this in documents, not so much in normal conversation).

For everyday spoken European Portuguese in a classroom or casual context, “Eu não tenho nenhuma dúvida agora” or simply “Não tenho dúvidas” is more natural.

Could I just say “Não tenho dúvidas” instead? Is there any difference?

Yes, you can, and it’s very common.

Não tenho dúvidas.
Literally: I don’t have doubts.

In context, it can mean:

No questions / All clear (in a class, explaining something)
- Teacher: Alguma dúvida? – Any questions?
- Student: Não, não tenho dúvidas. – No, I don’t have any (questions).
I’m certain / I have no doubt (about something)
- Não tenho dúvidas de que isto vai funcionar.
  I have no doubt this will work.

Your original sentence is slightly more explicit and time‑bound with agora (now).
In many situations, “Não tenho dúvidas” is enough and natural.

Can agora go in a different place, like “Agora, eu não tenho nenhuma dúvida”? Does the meaning change?

Yes, you can move agora, and the basic meaning (time: now) stays the same, but the focus can change a bit.

Possible orders:

Eu não tenho nenhuma dúvida agora.
Neutral: right now, I don’t have any questions.
Agora não tenho nenhuma dúvida.
Slightly more emphasis on “now (as opposed to before) I don’t have any questions.”
Agora, eu não tenho nenhuma dúvida.
Similar to the previous one; the comma and “eu” can add a bit of contrast:
“Now, I don’t have any questions.”

All are grammatically correct. The most “default” sounding versions are:

Agora não tenho nenhuma dúvida.
Eu não tenho nenhuma dúvida agora.

How would I say “I do have a question” (contrasting with “I don’t have any questions”)?

You’d normally use uma dúvida or uma pergunta:

Tenho uma dúvida. – I have a question.
Tenho uma pergunta. – I have a question.

To emphasise the contrast (“actually, I do have one”), you can add “até” or stress the eu:

Eu até tenho uma dúvida. – Actually, I do have a question.
Eu tenho uma dúvida, afinal. – I do have a question, after all.

What’s the difference between “alguma dúvida?” and “nenhuma dúvida”?

Alguma dúvida?
Literally: Any doubt?
Function: “Any questions?” – a very common way for teachers, trainers, etc. to ask.
Nenhuma dúvida.
Literally: No doubt / no doubts.
As a short answer: No questions.

Example exchange:

Teacher: Alguma dúvida? – Any questions?
Student: Não, nenhuma dúvida. – No, no questions.

How is nenhuma pronounced in European Portuguese?

In European Portuguese:

nenhuma is roughly: [nə-ˈɲu-mɐ]

Breakdown:

ne- → very reduced vowel, like a schwa: nə
-nhu- → nh = “ny” sound (like canyon): ɲu
-ma → final -a often sounds like ɐ (a very reduced “uh”)

So it’s closer to: nə-NYU-mɐ (said quickly and lightly).