Learn Finnish: Sinun täytyy laittaa käsimatkatavarat pois, ennen kuin laskeutuminen alkaa. - You must put the hand luggage away before the landing starts.

In Finnish, you must use a comma to separate a main clause from a subordinate clause if both have their own conjugated verbs. The main clause has täytyy and the subordinate clause has alkaa, so the comma before ennen kuin is mandatory.

Can I just say ennen laskeutuminen alkaa without kuin?

No. When "before" introduces a full clause that contains its own verb (like "the landing starts"), you must use the two-word conjunction ennen kuin. The word ennen by itself is only used with nouns in the partitive case (for example, ennen laskeutumista - "before the landing").

Why is it Sinun täytyy instead of Sinä täydyt?

This is the Finnish necessive structure. Verbs expressing necessity, like täytyy (must), require the person who has to do the action to be in the genitive case (sinun). The verb täytyy never conjugates for person; it always stays in the 3rd person singular and is followed by the basic infinitive of the action verb (laittaa).

Is the object käsimatkatavarat (hand luggage) affected by the special object rules for täytyy sentences?