Enciende los faros, que ya casi es de noche.

It is the affirmative tú command of encender.

So this is the form you use when telling one person you know well to turn something on:

• encender = to turn on, light, switch on
• tú enciendes = you turn on
• enciende = turn on

It also shows the normal stem change e → ie of encender.

A useful contrast:

• Enciende los faros = Turn on the headlights
• No enciendas los faros = Don’t turn on the headlights

Because Spanish usually leaves subject pronouns out when they are already clear from the verb form.

In enciende, the command already tells you the speaker is addressing tú, so tú is unnecessary.

You could say Tú enciende los faros, but that would sound more emphatic, as if you were contrasting that person with someone else.

Yes — it can mean both.

Context decides the meaning:

• un faro by the sea = a lighthouse
• los faros del coche = the car’s headlights

In this sentence, because someone is being told to turn them on as it gets dark, los faros clearly means headlights.

Los faros is more specific. It refers to the vehicle’s headlights.

Las luces is more general and can mean the lights of the car more broadly.

So:

• Enciende los faros = Turn on the headlights
• Enciende / pon las luces = Turn on the lights

In everyday speech in Spain, pon las luces is also very common.

Here que introduces the reason or explanation for the command.

So que ya casi es de noche means something like:

• because it’s almost night
• since it’s getting dark
• it’s nearly nighttime now

This is a very common spoken Spanish pattern, especially after commands:

• Ven, que hace frío = Come on, it’s cold
• Date prisa, que llegamos tarde = Hurry up, we’re arriving late

So this que is not that or which. It is explanatory.

Because the part after que is an explanatory clause: it gives the reason for the command.

So the structure is:

• Enciende los faros = command
• que ya casi es de noche = explanation

In writing, that comma is normal and helps show the pause:

Enciende los faros, que ya casi es de noche.

They add nuance.

• casi = almost
• ya = already, by now, at this point

Together, ya casi gives the idea that the situation has progressed and it is now very close to nightfall.

So it is not just it’s almost night. It is more like:

• it’s already almost night
• it’s getting pretty late now
• by now it’s nearly nighttime

If you removed ya, the sentence would still be correct, but it would lose some of that sense of urgency or progression.

Because es de noche is the normal fixed expression.

In Spanish, you commonly say:

• Es de día = It’s daytime
• Es de noche = It’s nighttime / It’s night

Using estar here is not the standard choice. If you want to use estar, you would normally say something slightly different, for example:

• Está oscuro = It’s dark

That is close in meaning, but not exactly the same expression.

Because de noche is part of an idiomatic time expression.

In this sentence:

• es de noche = it is nighttime

Spanish often uses de in expressions of this type:

• de día = by day / in daytime
• de noche = by night / at night / nighttime

So learners should treat es de noche as a set expression, rather than trying to translate each word literally.

Not necessarily.

Spanish uses the imperative more naturally than English in many everyday situations. In a practical context like driving, Enciende los faros sounds normal.

Also, adding que ya casi es de noche makes it sound more like a sensible instruction with a reason, not a harsh order.

Depending on tone of voice, it can sound:

• neutral
• practical
• caring
• urgent

So the grammar itself is not rude.

The rest of the sentence stays the same, but the command changes.

• Encienda los faros, que ya casi es de noche.
  Formal singular, speaking to usted

• Encended los faros, que ya casi es de noche.
  Informal plural in Spain, speaking to vosotros

• Enciendan los faros, que ya casi es de noche.
  Formal plural in Spain, or general plural in most of Latin America

So enciende specifically shows that the original sentence is addressed to one person informally.

Yes. A very everyday alternative would be:

• Pon las luces, que ya casi es de noche.

That is often how people would casually say it in a car.

Compared with that:

• Enciende los faros sounds a bit more specific
• Pon las luces sounds a bit more everyday and conversational

Both are natural, and the original sentence is perfectly good Spanish.