Questions & Answers about Oletko sinä vielä saunassa?
How do we form saunassa from sauna, and why doesn't it use -ssä?
To say 'in' a place, we add the inessive ending to the noun. sauna is a simple word whose core stem doesn't change (it doesn't gradate), so we just stick the ending directly on the end to get saunassa. It takes -ssa instead of -ssä because of vowel harmony: sauna has the back vowels a and u, so the ending must use the back vowel a.
What does the -ko at the end of Oletko do?
The -ko is a question particle. By attaching it to the verb olet (you are), you turn a regular statement into a yes/no question. Whenever you attach -ko or -kö to a verb, that verb must move to the very beginning of the sentence.
Is the pronoun sinä absolutely necessary here?
No, it is entirely optional. Because the verb olet already clearly tells us the subject is 'you', you can drop sinä and just ask Oletko vielä saunassa? Finns very often drop the pronouns for 'I' and 'you' in everyday speech.
Why does vielä come before saunassa?
The word vielä means 'still' or 'yet'. In Finnish, adverbs like this usually come right before the specific information they are emphasizing. Placing it before saunassa emphasizes the state of still being inside the sauna specifically.