Learn Finnish: Sinun on nolo unohtaa tämä kirjoittamaton ele. - It is embarrassing for you to forget this unwritten gesture.

This sentence uses the genitive experiencer structure. When evaluating an action by stating that it is a certain way for someone to do something, Finnish places the person experiencing the feeling in the genitive case (sinun). This is followed by the verb olla in the 3rd person singular, an adjective, and a 1st infinitive (unohtaa).

Why is the adjective nolo in the nominative case rather than the partitive noloa?

This is a strict rule of the genitive experiencer structure: the adjective must be in the nominative case. If you removed the person and just made a general statement, you would use the partitive (On noloa unohtaa). But as soon as you add the genitive experiencer, the adjective snaps into the nominative (Sinun on nolo unohtaa).

Why is the object tämä kirjoittamaton ele in the nominative form instead of the genitive-accusative?

They make up the direct object of the verb unohtaa. In Finnish, when the main clause of a sentence lacks a traditional nominative subject (which is exactly what happens in this evaluative structure), a total object takes the nominative-like accusative form. If a normal subject were present, the object would take the genitive form (tämän kirjoittamattoman eleen).

How does the word kirjoittamaton get its meaning of unwritten?