Serán las ocho, porque ya está oscuro fuera.

Here serán is not a simple future meaning they will be in the normal time sense. In Spanish, the future tense is often used to make a guess or express probability about the present.

So Serán las ocho means something like:

• It must be eight o’clock
• It’s probably eight
• I guess it’s eight

If you say Son las ocho, that means It is eight o’clock as a fact.

Grammatically, yes: serán is the third person plural future of ser.

But in this sentence, Spanish uses that form idiomatically for guessing about the present. Since clock times are expressed with son las..., the verb appears in the plural, so the future form is serán.

So:

• Son las ocho = It is eight o’clock
• Serán las ocho = It must be / it’s probably eight o’clock

Spanish tells time with the definite article:

• Es la una = It’s one o’clock
• Son las dos = It’s two o’clock
• Son las ocho = It’s eight o’clock

The article la/las is used because the expression is understood as referring to la hora or las horas.

So las ocho is the normal way to say eight o’clock.

Because the implied noun is horas, which is feminine plural.

So the full underlying idea is something like:

• Son las ocho horas

In real Spanish, horas is normally left out, but the article stays:

• las ocho

That is why it is feminine plural.

Porque means because.

In this sentence:

• Serán las ocho, porque ya está oscuro fuera.
• It must be eight, because it’s already dark outside.

The speaker is giving a reason for their guess.

A common learner point is not to confuse:

• porque = because
• por qué = why

So:

• ¿Por qué está oscuro? = Why is it dark?
• Está oscuro porque es tarde. = It is dark because it is late.

Ya here means already.

So ya está oscuro fuera means:

• it’s already dark outside

It suggests that the darkness is a clue that the time is later than expected, or that the change has happened by now.

Without ya, está oscuro fuera would simply mean:

• it’s dark outside

With ya, there is a stronger sense of by now / already.

Spanish often uses estar for states or conditions, especially temporary or changeable ones, and ser for more essential characteristics.

Here, dark outside is a current condition, so Spanish says:

• Está oscuro = It is dark

Compare:

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

So in your sentence, está is used because the speaker is describing the current state of the outside environment.

Yes, fuera and afuera can both mean outside, and in many contexts both are possible.

So these are both natural:

• Está oscuro fuera
• Está oscuro afuera

In Spain, fuera is very common and often sounds especially natural in this kind of sentence.

A small nuance:

• fuera can work very naturally as outside / outdoors
• afuera often feels a bit more explicitly locational, though the difference is not huge here

Está oscuro fuera is the most neutral, natural order here.

Spanish word order is flexible, but different orders can shift the emphasis.

• Está oscuro fuera = neutral: It’s dark outside.
• Fuera está oscuro = more emphasis on outside, perhaps contrasting with indoors
• Está fuera oscuro = generally not the natural order for this meaning

So the given version is simply the most standard way to say it.

Not really exactly. It suggests an estimate or inference, not certainty.

It usually means something like:

• It must be around eight
• I suppose it’s eight
• It’s probably eight

If you know the exact time, you would normally say:

• Son las ocho

So the future tense here adds uncertainty.

Yes, very common.

Spanish often uses the future tense to express probability or supposition about the present.

Examples:

• Estará en casa. = He’ll be at home / He’s probably at home.
• Tendrá unos treinta años. = He must be about thirty.
• Serán las ocho. = It must be eight o’clock.

This is a very useful pattern to recognise, because it does not always refer to the future in time.

Yes. Deben de ser las ocho is another natural way to express probability:

• Deben de ser las ocho = It must be eight o’clock

That said, Serán las ocho is shorter and very idiomatic.

A useful comparison:

• Son las ocho = certain
• Serán las ocho = probable / guessed
• Deben de ser las ocho = probable / inferred

All are correct, but they show different levels or styles of certainty.

Yes: it reinforces the standard pattern for clock time.

Spanish uses:

• Es la una for one o’clock
• Son las... for all other hours

So:

• Es la una
• Son las dos
• Son las ocho

And if you use the future of probability, the same pattern stays:

• Será la una = It must be one
• Serán las ocho = It must be eight

So this sentence is a good example of both telling time and using the future to guess.