Learn Finnish: Oletko sinä varma?

The suffix -ko (or -kö) is a particle used to form yes/no questions. By attaching it to olet (you are) and putting it at the start of the sentence, it changes the statement into the question 'Are you?'.

Why is the suffix -ko instead of -kö here?

This is due to vowel harmony. The word olet contains the back vowel o, so it requires the back vowel ending -ko. If a word contained front vowels like y, ä, or ö, you would use -kö instead.

Can I leave out the word sinä?

Yes, you can simply say Oletko varma?. Because the verb olet ends in -t, which always points to 'you' (singular), the pronoun sinä is optional. Including it just adds a little extra emphasis.

Since this lesson is about negation, how would I ask 'Are you not sure?'

To make a negative yes/no question, you attach the question particle to the negative verb et to make Etkö (Are you not). Then, you use the connegative form of the verb to be, which is ole. The full negative question becomes Etkö sinä ole varma?.