I concur with the origional resposne, only to add that of the top two, the lower one would be part of a more polite version of the same question.
For example
Could you tell me what you were doing, when the accident occurred? (also please note the corrected spelling of occurred)
For the bottom two, they are written in two different tenses. Why not try this,
I tried to change a lightbulb, because it had burnt out. (non specific, could be any light bulb)
or
I was trying to change the lightbulb that had burnt out, in the corner office. (specific)