Learn Turkish: O dönemde tarih öğretmeni kimmiş? - Who was apparently the history teacher in that era?

Kimdi uses the definite past tense, asking a direct "Who was it?". You would use it if you and the listener had firsthand experience of the situation. Kimmiş uses the reported/inferential past (-miş), which means "Who was it, apparently?". It is perfect for asking about historical facts, rumors, or asking what someone else has read or heard about an era they didn't personally live through.

Why is the question word kimmiş at the very end of the sentence?

In Turkish, question words usually sit in the exact spot where the answer will go. Because the answer to this question acts as the main predicate of the sentence (for example, "The teacher was Ali"), the question word kim takes the predicate position at the end and directly receives the "was" suffix (-miş).

Why is there an -i at the end of öğretmeni?

This is a quick refresher on noun compounds! When you combine two nouns to create a specific concept, the second noun must take a possessive suffix. So, tarih (history) + öğretmen (teacher) becomes tarih öğretmeni (literally "history its-teacher").

How does the phrase O dönemde break down?

O is the demonstrative for "that", dönem is the noun for "period" or "era", and -de is the locative case suffix meaning "in" or "at". Put together, it simply means "in that era".