Learn Hardcore Spanish (Latin America): Desde la azotea vemos un volcán lejano, pero casi nunca sale humo. - From the rooftop we see a distant volcano, but smoke almost never comes out.

Desde means from (starting point in space or time).
So desde la azotea = from the rooftop / from up on the roof, focusing on the point of origin of the view.

de la azotea could be understood, but it usually means of the rooftop (possession/origin), not the place where you are standing to see something.
en la azotea = on the rooftop, but it doesn’t by itself express the idea from there we see….

To talk about what you see from a certain place, desde is the natural preposition:

Desde mi casa veo el parque. – From my house I see the park.
Desde aquí no vemos nada. – From here we don’t see anything.

What exactly does azotea mean? Is it the same as techo or terraza?

Azotea (feminine noun) is a flat, usable rooftop, often where people can stand, hang clothes, sit, etc.

Differences:

azotea: the flat roof area people can go onto. Common in many Latin American cities, especially with flat-roofed buildings.
techo: the roof/ceiling in general (the covering of the building, or the interior ceiling). You usually don’t go to the techo.
terraza: terrace/patio/balcony. It might be on the roof or on a lower floor.

So desde la azotea suggests you are physically up on the flat rooftop of the building.

Why is it vemos and not miramos? Aren’t both “to see / to look”?

Spanish distinguishes more clearly between ver and mirar:

ver = to see (your eyes perceive something, sometimes without trying).
- Desde la azotea vemos un volcán lejano. – From the rooftop we see a distant volcano.
mirar = to look at / watch (a more active action, directing your eyes on purpose).
- Desde la azotea miramos el volcán. – From the rooftop we look at the volcano (we intentionally watch it).

In your sentence, vemos is natural because it just describes what is visible from that place, not necessarily that you’re deliberately staring at it.

Why is it un volcán lejano and not something with lejos, like un volcán lejos?

Lejano is an adjective: distant / faraway.
Lejos is an adverb: far / far away.

You can say:

un volcán lejano – a distant volcano (adjective + noun, like in English: distant volcano).
un volcán que está lejos – a volcano that is far away (verb + adverb).

But un volcán lejos is not correct, because you’re trying to use the adverb lejos directly as an adjective. For a simple noun phrase, un volcán lejano is the right structure.

Why is the adjective after the noun: un volcán lejano and not un lejano volcán?

By default, most adjectives in Spanish go after the noun:

un volcán lejano
una casa grande
un perro negro

You can say un lejano volcán, but putting the adjective before the noun often sounds:

more literary, poetic, or stylistic, or
emphasizes the quality (the “distantness”).

In everyday neutral speech, un volcán lejano is the normal order.

Why is there no subject pronoun nosotros in vemos? How do we know it means “we”?

In Spanish, the verb ending usually tells you the subject, so the pronoun is often omitted:

vemos = we see
veo = I see
ven = they/you all see

So Desde la azotea vemos… clearly means From the rooftop we see… even without nosotros.

You can say Nosotros vemos for emphasis or clarity, but in a normal context it sounds more natural to just say vemos.

Why is the present tense used: vemos and sale? In English we might say “we can see” or “it hardly ever gives off smoke”.

Spanish present indicative covers several uses that English splits into different forms:

Current action: Vemos el volcán. – We see / We are seeing the volcano.
Habitual action: Casi nunca sale humo. – Smoke almost never comes out.

You don’t need a special form like “is coming out” here; simple present sale already implies a general or habitual fact.

To stress an ongoing action, you can use the progressive:

Estamos viendo un volcán. – We are (right now) seeing a volcano.
Está saliendo humo. – Smoke is (right now) coming out.

But for a general description, vemos / sale in the simple present is the standard choice.

What does casi nunca mean exactly, and where does it go in the sentence?

Casi nunca literally means almost never; in practice it’s like hardly ever.

In pero casi nunca sale humo:

sale humo = smoke comes out
casi nunca modifies the verb, indicating frequency.

Common positions:

Casi nunca sale humo.
Humo casi nunca sale. (possible, but less common and more marked in tone)
You would not normally split it: casi sale nunca humo is wrong in standard Spanish.

Other frequency phrases:

nunca sale humo – it never gives off smoke.
casi siempre sale humo – it almost always gives off smoke.

Why is it sale humo? Why use salir with humo?

Salir literally means to go out / come out / leave, and it’s very common with things that emerge or come out of somewhere, including smoke:

Sale humo del volcán. – Smoke comes out of the volcano.
Sale agua de la tubería. – Water comes out of the pipe.
Sale un olor raro de la cocina. – A strange smell is coming from the kitchen.

In casi nunca sale humo:

the subject is humo (smoke),
sale is the verb (3rd person singular).

Spanish allows subject–verb order to change:

Casi nunca sale humo.
Casi nunca humo sale. (possible but unusual)
Humo casi nunca sale. (marked/emphatic)

The most neutral is the one you have: Casi nunca sale humo.

Why is there no article with humo? Why not el humo or un humo?

Humo is usually treated as a mass noun (like water, smoke, air in English). When you talk about it in general, you normally do not use an article:

Sale humo. – Smoke comes out.
Hay humo. – There is smoke.

You use el humo when you’re referring to specific, known smoke:

El humo del volcán es muy denso. – The smoke from the volcano is very thick.

Un humo is rare and would usually be poetic or very figurative (e.g. a kind of smoke, a cloud of smoke), not the normal way to say it here.

Could I say pero casi nunca hay humo instead of pero casi nunca sale humo?

Yes, pero casi nunca hay humo is grammatically correct and natural. The nuance changes slightly:

casi nunca sale humo puts the focus on the action of smoke coming out (the volcano doesn’t emit smoke very often).
casi nunca hay humo focuses more on the presence or absence of smoke (there is almost never smoke there).

In context, both can describe a volcano that rarely smokes, but sale humo emphasizes the emission.

Why is it pero and not sino in pero casi nunca sale humo?

Pero = but, used to introduce a contrast or exception.
Sino = but rather, used mainly to correct or replace a previous negative idea.

Examples:

Quiero ir, pero estoy cansado. – I want to go, but I’m tired. (contrast)
No quiero café, sino té. – I don’t want coffee, but rather tea. (correction after a no)

In your sentence, there is no direct negation that is being corrected; it’s just contrasting:

We see a distant volcano, but it almost never gives off smoke.

So pero is the correct conjunction here, not sino.