Learn Hardcore Dutch: Ik doe het licht aan als het buiten donker is. - I turn the light on when it is dark outside.

Aandoen is a separable verb made up of the verb doen and the prefix aan. In Dutch main clauses the finite verb (doe) must occupy the second position, so the prefix aan “falls off” and goes to the end: Ik doe … aan.

What’s the difference between aandoen and aanzetten when talking about lights?

Both can mean “to turn on” a light.
• Aandoen is more colloquial and also covers putting on clothes or applying something.
• Aanzetten is a bit more formal/technical, used for machines or devices (“zet de verwarming aan”, “zet de radio aan”).
In everyday speech either is fine for lights, but aandoen sounds more relaxed.

Why is the verb is at the end in the clause als het buiten donker is?

Because it’s a subordinate clause introduced by als (“when”). In Dutch subordinate clauses the finite verb moves to the end. In main clauses you’d see it second, but after als it drops to the back.

When should I use als versus wanneer to express “when”?

Use als for general, repeated or habitual situations (when it’s dark, when it rains). Wanneer is more common in questions (“Wanneer kom je?”, “When are you coming?”) or when referring to a specific point in time (“Wanneer het feest begint, …”).

Why do we say het licht instead of een licht, and could we use de lamp?

Het licht refers to illumination in general.
• Een licht would suggest “a light” as an unspecified light source.
• De lamp names the physical lamp or fixture.
If you want to emphasize the lamp itself you can say Ik zet de lamp aan, but het licht is what most Dutch speakers use when talking about switching on the light (illumination).

Can I omit buiten and simply say als het donker is?

Yes. Als het donker is (“when it’s dark”) is often enough because in most contexts it implies outside. You add buiten to be extra clear that you mean outside, not inside a room or a cellar.

Why is buiten placed before donker, and why doesn’t donker have an article or adjective ending?

Here buiten is an adverb meaning “outside,” not an adjective modifying donker. The structure is: het (dummy subject) + buiten (adverb) + donker (predicative adjective) + is. Predicative adjectives in Dutch (those that follow a form of zijn) are not inflected and don’t take articles.

Why use donker is instead of donker wordt, and how would that change the nuance?

Is describes a state: “when it is already dark.”
Wordt (“becomes”) focuses on the process of it getting dark: Ik doe het licht aan als het buiten donker wordt means “I switch on the light at the moment it starts getting dark.”

Can I start with the “when” clause, and if so, how does the word order change?

Yes. Fronting the subordinate clause gives you:
Als het buiten donker is, doe ik het licht aan.
After the comma the main clause inverts: the finite verb (doe) comes first, then the subject (ik), followed by the rest.