Learn Hardcore Spanish (Spain): El cielo está casi negro con tantas nubes. - The sky is almost black with so many clouds.

Estar is used for temporary or changing states, and for physical appearance that can change.
The sky is not always black; it’s just dark right now because of the clouds.
So El cielo está casi negro = The sky is (currently) almost black.
Ser would suggest a permanent or defining quality, which doesn’t fit normal sky color.

Could you also say El cielo es casi negro? When would that be used?

Yes, but it would mean something different and sounds unusual.
El cielo es casi negro would describe the normal or typical color of the sky in a place, for example: a planet, a painting, or a heavily polluted city, where the sky is usually almost black.
In everyday weather talk, you want a temporary state, so you use está.

What exactly does casi do here? Is it modifying está or negro?

Casi is an adverb meaning almost / nearly.
Here it modifies the adjective negro, softening it:

negro = black
casi negro = almost black, nearly black, not completely black
Grammatically, you can think of it as part of the adjective phrase casi negro after está.

Where can casi go in the sentence? Is El cielo casi está negro correct?

The natural position here is: El cielo está casi negro.
You generally put casi right before the adjective or adverb it modifies.

✅ El cielo está casi negro. (normal, natural)
❌ El cielo casi está negro. (sounds wrong or very strange in this context)

El cielo casi está negro would be interpreted as casi modifying está, and native speakers wouldn’t normally say it that way about color.

Why is negro masculine and singular? Shouldn’t it agree with nubes (which is plural)?

Adjectives agree with the subject of the verb, not with the cause of the state.
The subject is el cielo (masculine, singular), so:

el cielo → negro (masc. sg.)

Nubes is just part of the prepositional phrase con tantas nubes explaining why the sky looks that way; adjectives do not agree with nouns inside such phrases.

Why is cielo masculine?

Cielo is masculine because that’s its grammatical gender in Spanish: el cielo.
Most nouns ending in -o are masculine (el libro, el vaso, el cielo), although there are exceptions.
There isn’t a logical reason beyond usage and vocabulary; gender is something you generally just memorize with each noun.

Why is it tantas nubes and not muchas nubes? What’s the difference between tanto and mucho?

Both indicate a large quantity, but:

muchas nubes = many clouds / a lot of clouds
tantas nubes = so many clouds, so much (emphasizes a surprising or remarkable amount)

In this sentence, tantas nubes highlights that the number of clouds is great enough to almost turn the sky black.

Why does tantas end in -as and have an -s? How does tanto agree?

Tanto is a quantifier that agrees in gender and number with the noun it modifies:

masc. sg.: tanto dinero
masc. pl.: tantos coches
fem. sg.: tanta lluvia
fem. pl.: tantas nubes

Here, nubes is feminine plural (la nube / las nubes), so you must use tantas.

Why is there no article? Why not con las tantas nubes?

In Spanish, when you talk about an indefinite quantity of plural nouns (many clouds, so many clouds), you normally omit the article:

tantas nubes
muchas nubes
pocas nubes

Las tantas nubes would sound odd and would suggest a specific, known group of clouds that has already been clearly identified in the conversation. In general descriptions like this, you don’t use the article.

Could you say por tantas nubes instead of con tantas nubes?

You could say por tantas nubes, but the meaning changes slightly:

con tantas nubes = with so many clouds (they accompany the sky; descriptive)
por tantas nubes = because of / due to so many clouds (focus on cause)

Con tantas nubes sounds more neutral and descriptive for this kind of sentence.
Por tantas nubes emphasizes that the clouds are the reason the sky is almost black.

Is El cielo está casi negro por las nubes natural Spanish?

Yes, it’s understandable and grammatically correct:

El cielo está casi negro por las nubes. = The sky is almost black because of the clouds.

However, con tantas nubes is a bit more vivid and visual (you imagine the sky full of clouds), while por las nubes is more explicitly causal and less descriptive of quantity.

Can I use oscuro instead of negro? What’s the difference?

Yes, you can say: El cielo está casi oscuro con tantas nubes, though casi oscuro is a bit unusual (you’d more often hear muy oscuro).

negro = black (very intense darkness)
oscuro = dark (not necessarily black, just not bright)

Casi negro implies it’s extremely dark, nearly black.
Muy oscuro would suggest it’s very dark, but not to the point of being almost black.

Would speakers in Spain actually use this sentence in real life?

Yes. A Spaniard might say things like:

El cielo está casi negro con tantas nubes.
El cielo está casi negro, parece que va a llover.

It’s a natural, everyday way to describe a sky that looks very dark because it’s full of clouds.