What were you doing when the accident recurred?- This one is correct because it is a question and you have to have auxiliary verb(be-were) in front of a subject. Both of the sentences in the second set are correct, but depending on a context one is more appropriate than the other. Hope I helped :)

- What were you doing when the accident occurred? This is correct. - What you were doing when the accident ocurred? ======== - I tried to change a light bulb that had burnt out. Traté de cambiar una bombilla que se había quemado. - I was trying to change a light bulb that had burnt out. Estaba tratando de cambiar una bombilla que se había quemado. Both are correct.

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)