Doe het licht uit voordat je gaat slapen.

Doe is the imperative form of doen. In this sentence, it means do / turn off as a command or instruction.

So:

• doen = to do
• doe! = do! / turn off!

Because the full verb here is actually uitdoen, doe by itself is only part of the verb.

Because uitdoen is a separable verb.

The full verb is:

• uitdoen = to turn off

In many Dutch main clauses, and especially in commands, separable verbs split apart:

• Doe het licht uit.

So:

• doe = the conjugated part
• uit = the separable particle

This is very common in Dutch:

• Zet de radio aan.
• Maak de deur open.
• Trek je jas aan.

Not really. Here, uit is functioning as the separable particle of the verb uitdoen, not as an independent preposition meaning out.

So in this sentence, it is best to understand doe ... uit as one unit meaning turn off.

Yes. Uitdoen has more than one common meaning.

It can mean:

• to turn off
  
  • het licht uitdoen = turn off the light
• to take off
  
  • je jas uitdoen = take off your coat
• sometimes even to extinguish, depending on context

So Dutch often uses the same verb where English uses different verbs.

Because licht as a noun is a het-word in Dutch.

So the correct article is:

• het licht = the light

This is something you mostly just have to learn with the noun.

Be careful: licht can also be an adjective, meaning light in weight or color, but in this sentence it is a noun.

Dutch often uses het licht in the singular when talking about the room light or the light in general.

So:

• Doe het licht uit = Turn off the light

Even if English might sometimes say the lights, Dutch very naturally uses the singular here.

If you really mean multiple separate lights, you could say:

• Doe de lichten uit

But het licht uitdoen is the standard everyday phrase.

Yes, voordat is one word.

It means before and introduces a subordinate clause:

• voordat je gaat slapen = before you go to sleep / before you go to bed

It is a very common conjunction in Dutch.

Je is the unstressed form of jij.

Both can mean you, but:

• je = neutral, unstressed, very common in normal speech
• jij = stressed or emphasized

So here, voordat je gaat slapen is the normal, natural form.

You would use jij if you wanted emphasis or contrast, for example:

• voordat jij gaat slapen, moet je nog je tanden poetsen

Because voordat introduces a subordinate clause, and Dutch changes word order in subordinate clauses.

In a main clause, Dutch normally has the finite verb in second position. But after a conjunction like voordat, that rule no longer applies.

So you get:

• voordat
  • subject
    • rest of the verb phrase

Here:

• voordat
• je
• gaat slapen

This is normal Dutch subordinate-clause word order.

Gaan slapen means to go to sleep or to go and sleep / go to bed. It focuses on the action of starting sleep or going off to sleep.

So:

• voordat je gaat slapen = before you go to sleep / before you go to bed

If you said:

• voordat je slaapt

that sounds less natural here, because it means something more like before you are sleeping, which is not the usual way to express this idea.

In everyday Dutch, voordat je gaat slapen is the normal phrase.

Yes. Voor je gaat slapen is very common and natural in everyday Dutch.

So both are possible:

• voordat je gaat slapen
• voor je gaat slapen

In many situations, they mean the same thing. Voordat can sound a little more explicit or slightly more formal, but both are standard.

A polite version would usually be:

• Doet u het licht uit voordat u gaat slapen.

Here:

• u = polite/formal you
• doet u = polite imperative form

In informal Dutch, the original sentence is completely normal:

• Doe het licht uit voordat je gaat slapen.

Yes. A common alternative is:

• Schakel het licht uit.

That also means turn off the light.

The difference is mainly style:

• het licht uitdoen = very everyday, common
• het licht uitschakelen / schakel het licht uit = a bit more technical or formal

For normal daily speech, Doe het licht uit is probably the most natural choice.