Learn Turkish: Onu polis sanıyordum, meğer dolandırıcıymış. - I thought he was a police officer, it turns out he was a scammer.

While -miş is often used for things you heard from someone else, its other major job is expressing discovery or realization—meaning "I just found out" or "it turns out." In this sentence, it pairs perfectly with meğer (which means "it turns out" or "apparently") to emphasize your surprise at discovering his true identity.

The English translation says "I thought he was...", so why does the Turkish sentence start with Onu (him) instead of O (he)?

The verb sanmak (to suppose / to think) treats the person you are making an assumption about as its direct object. You are essentially saying "I supposed him [to be] a police officer." Because of this, the pronoun o must take the accusative case ending -nu, becoming onu.

Why use the past continuous sanıyordum instead of the simple past sandım?

Using the past continuous sanıyordum shows that this was an ongoing, continuous belief you held for a period of time in the past (like "I was under the impression..."). If you used sandım, it would sound more like a sudden, momentary thought that just popped into your head.

How is the word dolandırıcıymış built?