Learn Finnish: Eikö tuo tuttu mies ole opettaja? - Isn't that familiar man a teacher?

It combines the negative verb ei (he, she, or it is not) with the question ending -kö. In Finnish, you make a negative yes/no question by attaching -ko or -kö directly to the negative verb instead of the main verb.

Why do we use "ole" instead of "on" for the verb "to be" here?

Because the sentence is negative. The negative verb ei (inside eikö) already shows who is doing the action, so the main verb olla has to be in its stripped-down negative base form, which is ole.

The sentence is negative, so why isn't opettaja in the partitive case?

This is a common trap! In sentences where you say what someone or something is (an "A is B" sentence), the descriptive noun stays in the basic nominative case, even when negative. You use the partitive in negative sentences for objects (like "I don't see a teacher"), but not for simply describing who someone is.

Can tuttu mean both "familiar" and "a friend"?

It can be both! Here it is used as an adjective meaning familiar or known to the speaker (tuttu mies). However, Finns also frequently use it as a noun to talk about an acquaintance or a friend.