Questions & Answers about Sinun täytyy levätä, koska olet sairas.
Why does the sentence start with sinun (your) instead of the regular word for "you", sinä?
This is how Finnish expresses obligation! When using the verb täytyy (must / have to), the person who has to do the action must be in the genitive case (the "ownership" form). So, sinä becomes sinun. You can think of the structure as meaning "For you, it is necessary..."
If I wanted to say "I must rest" or "they must rest", does the verb täytyy change its ending?
No, täytyy is a special verb that is "frozen" in this form; it never changes to match the person. Instead of conjugating the verb, you just change the genitive pronoun in front of it. For example, "I must rest" is minun täytyy levätä and "he must rest" is hänen täytyy levätä.
Why is levätä in its basic dictionary form instead of having a personal ending?
In the täytyy sentence structure, the action that needs to be done is always put in the infinitive (the basic dictionary form). Since täytyy handles the "must" part, levätä just sits alongside it to mean "to rest".
In the second part of the sentence, why isn't the word for "you" included before olet?