Learn Hardcore Norwegian: Skilpadden trekker hodet sitt inn når den ser lyset. - The turtle pulls its head in when it sees the light.

In Norwegian you typically add the definite article as a suffix.
• skilpadde = “a turtle” (indefinite)
• skilpadden = “the turtle” (definite)
Here we’re talking about a specific turtle, so we use the definite form.

What does trekker mean? Could I use drar instead?

trekker literally means “pulls” or “draws” (in/transitive). In the phrase trekker hodet sitt inn, it conveys “withdraws” or “pulls back” the head.
You could say drar hodet sitt inn, but it’s less idiomatic. Native speakers almost always use trekker hodet inn for that reflexive, defensive motion.

Why is it hodet sitt and not hans hode or dens hode?

When the possessor and the subject of the sentence are the same, Norwegian uses a reflexive possessive pronoun (sin, sitt, sine) that agrees in gender and number with the thing possessed.
• hodet is a neuter noun, so we use sitt.
• sin would be used with masculine/feminine nouns, and sine with plurals.
Using hans or dens would imply “his” or “its” belonging to someone/thing else, not reflexively to the subject.

Why is inn placed after hodet sitt? Could it come earlier?

inn is a directional particle (“in/into”). With noun objects, Norwegian typically follows the pattern:
Verb + object + particle → trekker hodet sitt inn
If the object were a pronoun, you’d put the particle immediately after the verb:
trekker det inn (“pulls it in”)
You can occasionally see trekker inn hodet sitt, but the most common, neutral order is verb–object–particle.

What role does når play here? Does it always mean “when”?

når is a conjunction meaning “when” for general, habitual or future events. Examples:
• Når det regner, blir jeg inne. – “When it rains, I stay inside.”
For a single past event, you’d use da instead:
• Da jeg så lyset, ble jeg overrasket. (“When I saw the light, I was surprised.”)

Why do we need den in når den ser lyset? Can’t we drop it?

Every clause in Norwegian needs an explicit subject (except imperative). In the subordinate clause introduced by når, you must show who does the seeing. Here den refers back to skilpadden (“it”).

Why is the word order in når den ser lyset subject–verb–object, instead of verb–subject–object?

In Norwegian main clauses you follow V2 (verb-second) word order. But in subordinate clauses (those introduced by a conjunction like når), you do not invert; you keep the normal SVO order: subject → verb → object.

Why is lyset in the definite form instead of lys or et lys?

We’re talking about “the light” as a known phenomenon—not “a light” or “lights.” In Norwegian, to speak generally of all light or a specific light source already mentioned, you use the definite: lyset.