Learn Swedish: Han har långt hår i nacken. - He has long hair on the back of his neck.

When talking about body parts, Swedish usually uses the definite article (like nacken) rather than a possessive pronoun like hans (his). Because the context makes it obvious whose body part it is, saying "the neck" is the most natural way to express this in Swedish.

Why use nacken here instead of halsen for "neck"?

In Swedish, en nacke refers specifically to the back of the neck, which is where this hair grows. En hals refers to the front of the neck or the throat. English often just uses "neck" for both, but you need nacken here to paint the right picture!

Why do we use the preposition i (in) instead of på (on) for "on the back of his neck"?

Prepositions rarely translate directly between languages. In Swedish, hair is considered to be rooted "in" the nape, so you say i nacken. Using på nacken would sound a bit like the hair is an object just resting on top of his neck.

Why is the adjective långt instead of just lång?