Learn Finnish: Täytyykö sinun palata lokakuussa? - Do you have to return in October?

In Finnish, to say that something happens in a specific month, you add the inessive case ending -ssa or -ssä to the name of the month. Because lokakuu contains back vowels (o, a, u), vowel harmony means it takes -ssa, creating lokakuussa ("in October").

Why is the word for "you" in the genitive form sinun instead of the basic form sinä?

The verb täytyy ("have to" or "must") is a special type of verb called a necessity verb. In Finnish, the person who "has to" do the action must be put into the genitive case. So instead of sinä (you), we must use sinun. You can think of the literal structure as "Is it of you necessary to return?"

If the sentence is about "you" having to do something, why isn't the verb conjugated to match "you"?

Necessity verbs like täytyy are very strict: they do not take personal endings at all (like -n for "I" or -t for "you"). The verb always stays in this exact 3rd-person singular form (täytyy), no matter who has to do the action. The person doing the action is shown by the genitive word (sinun) instead.

What does the -kö at the end of Täytyykö do?