Skilpadden trekker hodet sitt inn når den ser lyset.

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.

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.

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.

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.

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.”)