Learn Turkish: Biyologlar tespit etti ki balık popülasyonu azalıyor. - Biologists determined that the fish population is decreasing.

You are right! Traditionally, Turkish uses the object participle -dığını (e.g., Biyologlar balık popülasyonunun azaldığını tespit etti). However, using ki is a very common alternative. It lets you structure the sentence exactly like English: Main Clause + ki + Noun Clause. It is often used in news, formal statements, or to avoid overly long and confusing participle chains.

Does using ki change the standard word order of the sentence?

Yes, it completely changes it. Standard Turkish puts the main verb at the very end. But when you use ki as a complementizer, you place the main clause first (Biyologlar tespit etti), followed by ki, and then the subordinate clause (balık popülasyonu azalıyor).

Why does Turkish use the present continuous azalıyor here, even though the main verb etti is in the past tense?

Turkish does not use 'sequence of tenses' exactly like English. Instead, it keeps the perspective of the original moment. At the time the biologists made their determination, the population was currently decreasing. Therefore, Turkish uses the present continuous -ıyor inside the ki clause, ignoring the past tense of the main clause.

Why is there an -u at the end of popülasyonu?