Learn Finnish: Tarvitset salaisen koodin avataksesi oven. - You need the secret code to open the door.

This is the "in order to" structure taught in this lesson. It is built from the first infinitive of the verb (avata, to open) + the translative marker (-kse-) + a possessive suffix matching the person doing the action. Since "you" are opening the door, we add the "your" suffix (-si), making avataksesi ("in order for you to open").

Why is the object oven (with an -n) instead of just ovi?

You might remember that infinitives can sometimes take a basic nominative object without an -n (like in minun täytyy avata ovi - I must open the door). However, the -kseen "in order to" structure does not follow that rule. It takes a standard total object with the -n ending, just like a regular active verb would.

Why is the object of the first clause salaisen koodin?

The verb tarvitset (you need) requires a standard object. Because the sentence is an affirmative statement where a specific person ("you") is doing the action, the object takes the -n ending. Both the adjective salainen and the noun koodi must get this ending.

Why does salainen change to salaise- before the -n is added?