Por la mañana había tanta niebla en la carretera que mi hermana encendió las luces antes de salir.

Yes. Por la mañana means in the morning.

In Spanish, time expressions like these are very common:

• por la mañana = in the morning
• por la tarde = in the afternoon / evening
• por la noche = at night / in the evening

A native English speaker may expect en la mañana, but in Spain por la mañana is the normal choice.

Había is the imperfect form of haber. Here it is used to describe a background situation: there was fog on the road.

Spanish often uses:

• imperfect for background, description, ongoing conditions
• preterite for completed actions/events

So in this sentence:

• había tanta niebla = background condition
• encendió las luces = the specific action that happened because of that condition

If you said hubo, it would sound more like a bounded event, not the general scene-setting description.

Exactly. In this sentence, había means there was.

This is the impersonal use of haber, which is used to express existence:

• hay = there is / there are
• había = there was / there were
• hubo = there was / there were (preterite)

So:

• Había niebla = There was fog

Even though niebla is singular here, haber stays in the same impersonal form.

Because tanto/a/os/as is used before a noun.

Here, niebla is a feminine singular noun, so the form must agree with it:

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

So:

• tanta niebla = so much fog

By contrast, tan is used before adjectives or adverbs:

• tan espesa = so thick
• tan rápido = so quickly

It means so much ... that.

This is a very common Spanish pattern for expressing cause/result:

• Había tanta niebla... que...
• There was so much fog... that...

Other examples:

• Tenía tanto sueño que se durmió. = He was so sleepy that he fell asleep.
• Hacía tanto frío que no salimos. = It was so cold that we didn’t go out.

So the sentence is built like this:

• cause/intensity: había tanta niebla en la carretera
• result: que mi hermana encendió las luces antes de salir

Because encendió refers to a completed action: she turned on the lights.

The sentence contrasts:

• había = ongoing background condition
• encendió = one completed action

This is a very typical combination in Spanish storytelling:

• imperfect for what was happening
• preterite for what happened

So: the fog was there, and then she turned on the lights.

It comes from encender, which means to turn on, to switch on, or to light.

Some forms:

• encender = to turn on
• enciendo = I turn on
• encendía = I/he/she was turning on; used to turn on
• encendió = he/she turned on

In this sentence:

• mi hermana encendió las luces = my sister turned on the lights

Because in Spanish, vehicle lights are often referred to in the plural: las luces.

Depending on context, this could mean:

• the headlights
• the car lights
• the lights in general

English sometimes says the lights too, so this matches quite naturally here.

En la carretera here means on the road or on the highway/roadway as a location.

• en la carretera focuses on where the fog was
• por la carretera would more strongly suggest movement along the road

Since the sentence is describing a condition located there, en la carretera is the natural choice.

After antes de, Spanish uses the infinitive when the subject is the same.

So:

• antes de salir = before leaving

Here, the understood subject of salir is the same person: mi hermana.

Compare:

• Mi hermana encendió las luces antes de salir.
  = My sister turned on the lights before leaving.
  (same subject)

But if the subject changes, Spanish often uses antes de que + subjunctive:

• Mi hermana encendió las luces antes de que saliéramos.
  = My sister turned on the lights before we left.

By default, it refers to mi hermana, the subject of the main clause.

So the meaning is:

• my sister turned on the lights before leaving

Spanish often leaves this understood rather than repeating the subject.

Yes. Salir is a broad verb and can mean:

• to leave
• to go out
• to head out
• to set off

In this sentence, because of the road and the car lights, it most naturally suggests something like:

• before setting off
• before leaving in the car

So antes de salir is very natural and flexible.

Spanish often puts the setting/background first, especially in narration.

So this order sounds very natural:

• Por la mañana = sets the time
• había tanta niebla en la carretera = sets the scene
• que mi hermana encendió las luces antes de salir = gives the main action/result

If you started with Mi hermana, the sentence would focus more on her. As written, it first creates the atmosphere.

Yes, but the meaning changes slightly.

• Había mucha niebla = There was a lot of fog
• Había tanta niebla que... = There was so much fog that...

Tanta strongly sets up a consequence. It makes the second part feel more natural because it introduces a result:

• so much fog that she turned on the lights

Yes. Niebla is a feminine noun:

• la niebla
• mucha niebla
• tanta niebla
• la niebla espesa

That is why the sentence uses tanta, not tanto.

Yes:

• Por la mañana = In the morning
• había = there was
• tanta niebla = so much fog
• en la carretera = on the road
• que = that
• mi hermana = my sister
• encendió = turned on
• las luces = the lights
• antes de salir = before leaving / before setting off

A very literal version would be:

• In the morning there was so much fog on the road that my sister turned on the lights before leaving.

Yes, it is an excellent example.

It shows a very common pattern in Spanish:

• imperfect = background description
  había tanta niebla
• preterite = main completed event
  encendió las luces

This is one of the most important contrasts for learners to notice. The fog is the situation already in progress; turning on the lights is the action taken in response.