Learn Finnish: Onko tausta sinun takanasi valkoinen vai vihreä? - Is the background behind you white or green?

When a postposition is used with a personal pronoun like sinun (your), the postposition must take a possessive suffix that matches the pronoun. Because the pronoun is sinun, we add the second-person singular suffix -si to the postposition takana, making it takanasi.

Could I just say takanasi and leave out the word sinun?

Yes, you absolutely can! In standard Finnish, the first and second person pronouns like minun and sinun can be dropped because the possessive suffix on the postposition already tells us who it belongs to. Saying just takanasi without the pronoun is completely correct and very common.

Why does the sentence use vai for "or" instead of tai?

In Finnish, vai is used specifically in questions when you are presenting alternatives and asking the listener to choose exactly one of them. Because the background can only be white or green, but not both, we must use vai. The word tai is used in regular statements or when multiple options could be true at the same time.

How does the word onko create a question here?

Onko is the verb on (is) with the question particle -ko attached to it. To form a question in Finnish without a specific question word like what or where, you add -ko or -kö to the word you are asking about and move it to the very beginning of the sentence.