"Bu ne zaman oldu?"

Translation: When did this happen?

[G0108]

Why can't it be "When was this?"

[ex_contributor]

I don't think they are the same, that's rather "bu ne zamandı?"

[DottyEyes]

I agree. In English, if we were asking when a past event occurred, we could use either "When was this?" or "When did this happen?" The difference might be "When was this?" is more common for questions about repeated things. For example: Sally: "We used to take the trolley into town." John: "When was this?"

[maguskrool]

I wrote "what time was this?", is there any reason this is wrong? To me it was the most direct translation.

[Jannat2727]

Doesnt oldu also mean dead?

[DottyEyes]

You need the diacritic on the "o" and then also on the "u," for vowel harmony.
O öldü (it died).
O oldu (it happened).

[Peet_S]

How about: "This happened when?"