When I ________by the station , I saw the accident_________.
A past;happening
B passed; happend
C past ;to happen
D passed;hapen

When I ________by the station , I saw the accident_________.
A past;happening
B passed; happend
C past ;to happen
D passed;hapen

选D，楼上两位肯定错的。
第一个空要填动词，passed是pass的过去式，past：prep.越过, 晚于；vbl.pass的过去分词。
happen不及物，没有被动形式。（你打错了，D少打一个p，B少打一个e）

hear/see/watch/notice/observe/find/feel/look at/listen to + sb./sth. + do/doing/done

do强调的是（看到/听到……）全过程
doing强调的是（看到/听到……）正在做
done强调的是被动

“see sb. do”的被动是“sb. be seen to do”
“see sb. doing”的被动是“sb. be seen doing”
“see sth. done”的被动是“sth. be seen done”
“make sb. do”的被动是“sb. be made to do”