Nunca había visto tanta niebla en el valle.

Había visto is the pluperfect (also called the past perfect): had seen.

It is used when you are looking back from a point in the past and saying that something had never happened up to that point.

So:

• Nunca vi tanta niebla en el valle = I never saw so much fog in the valley
  This sounds like a simple past statement.
• Nunca he visto tanta niebla en el valle = I have never seen so much fog in the valley
  This connects more directly to the present.
• Nunca había visto tanta niebla en el valle = I had never seen so much fog in the valley
  This implies a past reference point: for example, when I arrived there that morning, or before that day, I had never seen that much fog.

So the sentence is choosing a viewpoint from the past.

It is made with:

• haber in the imperfect: había
• the past participle of ver: visto

So:

• había visto = had seen

This is the pattern for the pluperfect:

• yo había visto
• tú habías visto
• él/ella había visto
• nosotros habíamos visto
• vosotros habíais visto
• ellos/ellas habían visto

The past participle visto is irregular, so it does not come from a regular -er/-ir pattern like comido or vivido.

Both are possible.

• Nunca había visto tanta niebla en el valle
• No había visto nunca tanta niebla en el valle

They mean essentially the same thing: I had never seen so much fog in the valley.

The important rule is:

• If a negative word like nunca, nadie, or nada comes before the verb, you do not add no.
• If it comes after the verb, you normally use no before the verb.

So:

• Nunca había visto... ✔
• No había visto nunca... ✔
• No nunca había visto... ✘

Starting with Nunca is very natural and gives the sentence a slightly stronger, more emphatic feel.

Because tanta goes with a noun, while tan usually goes with an adjective or adverb.

Here:

• niebla is a noun
• so you need tanta: tanta niebla = so much fog

Compare:

• Había tanta niebla = There was so much fog
• Estaba tan oscuro = It was so dark

So:

• tan + adjective/adverb
• tanto/tanta/tantos/tantas + noun

Because tanta has to agree with niebla, and niebla is:

• feminine
• singular

So the correct form is:

• tanta niebla

Agreement pattern:

• tanto
  • masculine singular noun
• tanta
  • feminine singular noun
• tantos
  • masculine plural noun
• tantas
  • feminine plural noun

Examples:

• tanto tiempo = so much time
• tanta niebla = so much fog
• tantos coches = so many cars
• tantas casas = so many houses

Because niebla here is being used as an uncountable noun, like fog in English.

You do not normally count fog as separate units. You treat it as a mass, so Spanish uses the singular:

• mucha niebla
• tanta niebla

This works like:

• mucha agua = a lot of water
• mucho tráfico = a lot of traffic
• mucho humo = a lot of smoke

So tanta niebla is perfectly natural.

Because en el valle means in the valley, which gives the location.

• en el valle = in the valley

If you said del valle, that would usually mean of the valley or from the valley, which is a different relationship.

Compare:

• Había niebla en el valle = There was fog in the valley
• La niebla del valle = the fog of the valley / the valley’s fog

So in your sentence, en el valle is the natural choice because the valley is the place where the fog is being seen.

Yes. Spanish allows some flexibility in word order.

These are all possible:

• Nunca había visto tanta niebla en el valle.
• En el valle, nunca había visto tanta niebla.
• No había visto nunca tanta niebla en el valle.

They all express basically the same idea, but the emphasis changes a little.

For example:

• Nunca había visto... strongly highlights never
• En el valle... highlights the place
• No había visto nunca... is a more neutral negative structure

The original version is very natural and idiomatic.

Most of the time, yes, especially in a sentence like this.

• Nunca había visto... = I had never seen...

But in some contexts, especially in questions or with certain expressions, nunca can feel closer to ever.

For example:

• ¿Has visto nunca algo así? = Have you ever seen anything like that?

So:

• in statements like this one, nunca = never
• in some questions, it can correspond to ever

Because when a past participle is used with haber, it does not agree in gender or number with the subject.

So you always get:

• he visto
• había visto
• habíamos visto

not forms like vista or vistos in this structure.

Compare:

• La he visto = I have seen her
  Here visto stays the same.
• La puerta está abierta = The door is open
  Here abierta is an adjective, so it agrees with puerta.

So in había visto, visto stays unchanged.

Yes, but the meaning changes slightly.

• mucha niebla = a lot of fog
• tanta niebla = so much fog

Tanta often adds more emphasis, usually implying surprise, intensity, or comparison with what is normal or expected.

So:

• Había mucha niebla en el valle = There was a lot of fog in the valley
• Nunca había visto tanta niebla en el valle = I had never seen so much fog in the valley

The original sentence sounds stronger and more expressive.

The written accent shows the stress:

• ha-BÍ-a

It also helps indicate that the í forms a separate syllable from the a. So había has three syllables:

• ha - bí - a

Without the accent, Spanish pronunciation rules would suggest a different stress pattern, so the accent is necessary.

Yes, completely natural.

Everything in the sentence is standard and widely used in Spain:

• Nunca at the beginning sounds natural
• había visto is a normal pluperfect form
• tanta niebla is idiomatic
• en el valle is the expected prepositional phrase

A native speaker in Spain would find this sentence entirely normal.