Learn Swedish: Ingen lade märke till hans nya frisyr.

While both can translate to "noticed," they are used differently. Lägga märke till (past tense: lade märke till) means to visually observe or pay attention to a specific detail, like a haircut or a new shirt. Märka is usually more about sensing, feeling, or detecting something, like noticing that it has gotten cold or that someone is in a bad mood.

Why do we use hans here instead of the reflexive pronoun sin?

The reflexive pronoun sin always refers back to the subject of the same clause. The subject here is Ingen (Nobody). If you used sin, the sentence would mean "Nobody noticed their own haircut." Since we are talking about a different person's haircut, we have to use hans.

Why is the adjective nya in the definite form, but the noun frisyr in the indefinite form?

Possessive pronouns like hans make the noun phrase definite, so the adjective takes the definite ending -a (nya). However, a strict rule in Swedish is that a noun immediately following a possessive pronoun must be in the indefinite form. So it is hans nya frisyr, never hans nya frisyren.

Could I say Inte någon lade märke till instead of using Ingen?