Questions & Answers about Sinun täytyy harjata tukka nyt.
Why is tukka (hair) in the basic nominative form instead of tukan?
This is the special rule for necessity sentences! Normally, a total object (doing a complete action to one whole thing) takes the genitive-like ending (-n). But when the main verb expresses necessity, like täytyy (must), a singular total object is left in the basic nominative form (tukka).
Why is the subject sinun in the genitive case?
Necessity verbs like täytyy (must) and pitää (have to) always require the person who "must" do the action to be in the genitive case. That is why the basic word sinä (you) changes to sinun.
Wait, sinun usually means "your". How do we know it means "you must" here and not "your hair"?
Because of the word order and the grammar of necessity verbs. Sinun täytyy is the standard structure for "you must". If the sentence wanted to specify "your hair", it would usually attach a possessive suffix to the object, like sinun tukkasi.
Why doesn't the Finnish sentence just say sinun tukkasi for "your hair"?
When talking about doing everyday actions to your own body parts or clothing, Finnish usually drops the possessive words if the context is obvious. Simply saying harjata tukka is the most natural way to say "brush your hair" in this situation.