Cuanto más oscuro está, más brilla el reflector.

This is a very common Spanish comparison pattern. It works like:

cuanto + comparative..., más + comparative/result...

It corresponds to English the more..., the more... or the X-er..., the more...

So here:

Cuanto más oscuro está, más brilla el reflector.

follows the pattern:

• Cuanto más oscuro está = the darker it is
• más brilla el reflector = the more the lamp/spotlight shines

You may also see tanto in the second half:

Cuanto más oscuro está, tanto más brilla el reflector.

That is correct too, but in everyday Spanish the tanto is often left out.

Because this cuanto is not a question word or an exclamation.

Spanish uses the accented form cuánto in questions and exclamations, such as:

• ¿Cuánto cuesta?
• ¡Cuánto ruido!

But in your sentence, cuanto is part of a comparative structure, so it has no accent:

• Cuanto más estudias, más aprendes.

That is the same reason it is cuanto here.

Because estar is used for a state or condition, and here darkness is being treated as a current condition of the surroundings.

So:

• está oscuro = it is dark / it has become dark / it’s dark right now

If you used es oscuro, it would usually sound like you are describing something as inherently dark in color, style, or character, not talking about the current level of light.

For example:

• La habitación está oscura. = The room is dark right now.
• El color es oscuro. = The color is dark.

Spanish does not need a dummy subject like English it in expressions like it is dark.

So English says:

• It is dark

But Spanish simply says:

• Está oscuro

There is no need to add a pronoun. Spanish often leaves the subject unstated when it is obvious, unimportant, or just not expressed the way English does.

In a sentence like this, you can think of the idea as referring to the environment / the sky / the surroundings, but Spanish usually does not spell that out.

In expressions like está oscuro, Spanish normally uses the masculine singular form when no specific noun is stated.

You can think of it as matching an implied idea such as:

• el ambiente
• el cielo
• el lugar

All of those are masculine singular.

If you mention a specific feminine noun, then the adjective changes:

• La habitación está oscura.
• La calle está oscura.

But when Spanish just says está oscuro with no noun, oscuro is the normal form.

Because más is modifying the verb brilla.

Here, más brilla means something like shines more.

In this comparative pattern, Spanish often puts más before the verb to mirror the first clause:

• Cuanto más oscuro está, más brilla el reflector.

That creates a neat parallel structure.

You could also rearrange the second part and say:

• Cuanto más oscuro está, el reflector brilla más.

That is also understandable and natural. The original version just sounds a bit more stylistically balanced.

Because the subject is el reflector, which is singular.

• el reflector brilla = the reflector/spotlight shines

If the subject were plural, then the verb would be plural too:

• los reflectores brillan

So the verb form simply agrees with its subject.

In much of Latin America, reflector often refers to a spotlight, floodlight, or another strong directed light source. In some contexts it can even refer to a headlight.

So it does not always mean the same thing as English reflector, which often suggests something that only reflects light rather than produces it.

In this sentence, the most natural interpretation is probably a light source such as a spotlight or strong lamp.

Yes. Spanish allows some flexibility.

The original sentence is:

Cuanto más oscuro está, más brilla el reflector.

You could also say:

El reflector brilla más cuanto más oscuro está.

Both are correct. The original version puts the two comparative parts side by side, which makes the pattern especially clear.

Spanish often changes word order for emphasis, rhythm, or style, as long as the grammar is still clear.

In many parts of Latin America, yes.

You may hear:

• Mientras más oscuro está, más brilla el reflector.
• Entre más oscuro está, más brilla el reflector.

These are common in speech in some regions. However, cuanto más... más... is the safest neutral choice and is widely accepted in standard Spanish.

So if you are learning a general Latin American Spanish, cuanto más is an excellent form to know and use.

The comma separates the two parts of the comparative structure:

• Cuanto más oscuro está
• más brilla el reflector

This is very common in writing because it makes the sentence easier to read.

In short, the comma helps mark the pause between the two linked ideas: the condition and the result.