No tengo ni idea.

Ni adds emphasis. In this sentence, No tengo idea already means I have no idea, but No tengo ni idea sounds stronger, more like:

• I have absolutely no idea
• I have not the slightest idea

So ni is helping intensify the negative meaning.

Yes, Spanish often uses what English speakers think of as double negatives, and that is completely normal.

In No tengo ni idea:

• No negates the verb tengo
• ni idea adds emphasis within that negative structure

This is standard Spanish, not incorrect Spanish. In fact, saying both sounds natural.

Compare:

• No tengo idea = I have no idea
• No tengo ni idea = I have absolutely no idea

Yes. No tengo idea is completely correct and natural.

The difference is mainly tone:

• No tengo idea = neutral
• No tengo ni idea = stronger, more emphatic

So if you want to sound a little more expressive, use ni.

Because Spanish expresses this idea with the verb tener (to have), not to be.

So Spanish says literally:

• No tengo idea = I don’t have an idea

That is just the natural Spanish structure. English uses I have no idea too, so this part actually matches English pretty well.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• Tengo = I have

So No tengo ni idea already clearly means I have absolutely no idea.

You could say Yo no tengo ni idea, but adding yo usually gives extra emphasis or contrast, such as:

• Yo no tengo ni idea, pero ella sí sabe.
  I have no idea, but she does know.

Yes, idea is a feminine noun: la idea.

But in this expression, Spanish normally says:

• No tengo ni idea

not usually:

• No tengo ni una idea

Even though idea is countable in some contexts, this expression works as a fixed idiomatic phrase. It is best to learn it as a chunk: no tener ni idea.

It is common, natural, and fairly informal-neutral. It is not vulgar or rude. You can use it in everyday conversation very easily.

Examples:

• with friends: completely natural
• at work: usually fine in casual conversation
• in very formal situations: you might choose something softer, like No lo sé or No estoy seguro/a

So No tengo ni idea is everyday spoken Spanish, but not offensive.

Some common alternatives are:

• No sé. = I don’t know.
• No lo sé. = I don’t know it / I don’t know.
• No tengo idea. = I have no idea.
• No tengo la menor idea. = I don’t have the slightest idea.
• Ni idea. = No idea.

In conversation, Ni idea is very common and natural as a shorter version.

Yes, very often. Ni idea is a very common short reply.

Example:

• ¿Dónde está Carlos?
• Ni idea.

That sounds like:

• No idea
• I have no idea

It is a bit shorter and more conversational than the full sentence No tengo ni idea.

A simple Latin American pronunciation guide would be:

no TENG-go ni i-DE-a

A few notes:

• tengo has the stress on TEN
• idea has the stress on DE
• the h sound does not exist here; everything is pronounced clearly
• the two vowels in ni idea run together smoothly in fast speech

A rough English-friendly approximation is:

noh TEN-go nee ee-DEH-ah

Yes, the verb tener changes depending on the subject.

Examples:

• No tengo ni idea. = I have absolutely no idea.
• No tienes ni idea. = You have absolutely no idea.
• No tiene ni idea. = He/She/You-formal have(s) absolutely no idea.
• No tenemos ni idea. = We have absolutely no idea.
• No tienen ni idea. = They/You-plural have absolutely no idea.

The rest of the phrase stays the same.

It can be used both literally and loosely.

Literal:

• ¿Sabes cuándo llega el bus?
• No tengo ni idea.
  = You really do not know.

Looser / conversational:

• ¿Por qué hizo eso?
• No tengo ni idea.
  = I really can’t guess / I have no clue.

So it can express both I don’t know and I can’t even guess.