Learn Hardcore Spanish (Spain): Durante el incendio, el cielo se llenó de humo gris y olía muy mal. - During the fire, the sky filled with gray smoke and it smelled very bad.

In Spanish, as in English, it’s very common (though not absolutely mandatory) to put a comma after an introductory phrase to separate it from the main clause:

Durante el incendio, el cielo se llenó de humo gris…
After the fire, the sky filled with gray smoke…

So the comma just marks a pause and makes the sentence clearer; it doesn’t change the grammar or meaning.

Why do we say «Durante el incendio» and not just «Durante incendio»? Why is «el» needed?

In Spanish, singular countable nouns almost always need an article (or another determiner like este, mi, etc.).

«Incendio» is a countable noun, so you normally say:

el incendio – the fire (a specific one)
un incendio – a fire (any fire)

Saying «Durante incendio» without an article sounds incomplete or incorrect in standard Spanish.

Similarly, el cielo is used rather than just cielo:

el cielo – literally the sky, but often translated as just the sky in English, where we can drop the article more often than Spanish does.

What exactly does «incendio» mean, and how is it different from «fuego»?

Both relate to fire, but they’re used differently:

fuego – fire in a general sense (flames, the element, heat).
- Hay fuego en la chimenea. – There is fire in the fireplace.
incendio – a large, destructive fire, like a building fire, forest fire, house fire.
- El incendio destruyó el edificio. – The fire destroyed the building.

In this sentence, «Durante el incendio» clearly refers to a serious fire, not just flames from a small source.

What does the «se» in «el cielo se llenó» do? Is it reflexive?

Here «se llenó» is a pronominal / middle form rather than a truly reflexive one.

llenar algo – to fill something
- El humo llenó el cielo. – The smoke filled the sky. (active voice)
llenarse – to become filled / to fill up (the subject undergoes the change)
- El cielo se llenó de humo. – The sky filled with smoke.

So «se» gives a meaning like “became filled”, focusing on the sky undergoing the change, not on who/what caused it. It’s similar to English using an intransitive verb: the room filled up.

Why is it «se llenó de humo» and not «se llenó con humo»?

With llenar(se), Spanish normally uses «de» to introduce what something gets filled with:

llenar un vaso de agua – to fill a glass with water
La plaza se llenó de gente. – The square filled with people.
El cielo se llenó de humo. – The sky filled with smoke.

«Con» is possible in some contexts, but «llenarse de» is the standard, idiomatic structure for this meaning. «Se llenó con humo» would sound unusual or foreign in this sentence.

Why is «se llenó» in the preterite but «olía» is in the imperfect? Why not both in the same tense?

This is a classic aspect contrast in Spanish:

«se llenó» (preterite of llenarse) shows a completed event:
The sky became filled at a certain moment (or over a limited period) during the fire.
«olía» (imperfect of oler) describes a background, ongoing state:
The smell was bad during that time; it’s not presented as a single, completed event.

So the structure is:

Durante el incendio,
- el cielo se llenó de humo gris (something that happened / change of state),
- y olía muy mal (continuous background situation).

If you used «olió» (preterite: it smelled), it would sound like you’re treating the bad smell as a punctual event, which is odd for smells.

Could we say «el cielo estaba lleno de humo gris» instead of «el cielo se llenó de humo gris»? What’s the difference?

Both are correct, but they emphasize different things:

el cielo se llenó de humo gris
Focuses on the process / change: the sky became filled with gray smoke.
el cielo estaba lleno de humo gris
Describes the resulting state: the sky was full of gray smoke (without focusing on how it got that way).

So:

Durante el incendio, el cielo se llenó de humo gris.
→ narrative of what happened.
Durante el incendio, el cielo estaba lleno de humo gris.
→ description of what things were like during that time.

Both fit; the original has a more “storytelling” feel.

Why is there no article before «humo gris»? Why not «del humo gris»?

Here «humo gris» is an uncountable substance, used in a general sense:

humo – smoke in general.
El cielo se llenó de humo gris. – The sky filled with gray smoke.

Spanish normally does not use an article with mass nouns in this kind of “filled with X” construction:

Se llenó de agua. – It filled with water.
Se llenó de nieve. – It filled/covered with snow.
Se llenó de humo. – It filled with smoke.

If you say «del humo gris», you’re specifying some particular smoke already mentioned or known, like:

El cielo se llenó del humo gris que salía de la fábrica.
– The sky filled with the gray smoke that was coming from the factory.

In the original sentence, the smoke is just introduced for the first time, so no article is natural.

Why is the adjective «gris» placed after «humo»? Could it go before?

The default position for adjectives in Spanish is after the noun:

humo gris – gray smoke
cielo azul – blue sky
agua fría – cold water

You can sometimes place adjectives before the noun, but that often adds a special nuance (more poetic, emotional, or emphasizing a subjective quality):

el gris humo – would sound poetic/literary, not neutral.

In everyday Spanish, «humo gris» is the normal word order.

Why isn’t it «grisa» instead of «gris»»? Doesn’t the adjective need to agree with «humo»?

It does agree, but «gris» is one of the adjectives that has one form for masculine and feminine in the singular:

Singular: gris – masculine or feminine
- humo gris (masculine)
- ropa gris (feminine)
Plural: grises
- nubes grises – gray clouds

So there is no «grisa» in correct Spanish; the correct singular form is always gris.

Why is it «olía muy mal» and not «olía muy malo»?

Because here we describe how it smelled, i.e. the manner, so we need an adverb, not an adjective.

mal is an adverb: badly, poorly, in a bad way.
malo/mala is an adjective: bad (as a quality of a noun).

After oler when you describe the smell itself, you use mal:

Huele mal. – It smells bad.
Olía muy mal. – It smelled very bad.

«Olía muy malo» would sound wrong in standard Spanish; it would be like saying “it smelled very bad (person)” — grammatically mismatched.

Also note: muy modifies adjectives and adverbs (muy mal, muy bueno), while mucho modifies verbs and quantities (llueve mucho, mucho humo).

Could we say «olía a humo» instead of «olía muy mal»»? What’s the difference?

Yes, but the meaning changes:

olía a humo – it smelled like smoke
→ describes the type of smell (smoky).
olía muy mal – it smelled very bad
→ describes the quality of the smell (unpleasant).

You can also combine them:

Olía muy mal, a humo. – It smelled very bad, like smoke.

In the original sentence, «olía muy mal» emphasizes how unpleasant the smell was, not just that it was smoky.

How do you pronounce «olía», and what does the accent mark do?

«Olía» is pronounced roughly [oh-LEE-ah]:

o – like o in no
lí – stressed syllable, like lee
a – like a in father

The accent on í shows the stress is on li: o-LÍ-a.

Without the accent (olia), the natural stress by spelling rules would be on the o (first syllable), which would be incorrect for this verb form. The accent mark both:

Marks the correct pronunciation, and
Distinguishes «olía» (imperfect: was smelling / used to smell) from other forms like «olia» (which would be a misspelling).

Why is it «Durante» and not «Mientras»? Aren’t both “during/while”?

They are related but used differently:

durante + noun
- Durante el incendio – During the fire
- Durante la noche – During the night
mientras + verb (a clause)
- Mientras ardía el edificio, el cielo se llenó de humo gris.
  – While the building was burning, the sky filled with gray smoke.

So:

Use durante when it’s followed by a noun phrase.
Use mientras when it’s followed by a verb / sentence.

That’s why «Durante el incendio» is correct here.