Re: Comma and which
The first three are correct, though 'complete' in #3 needs to be 'completed'.
#4 is not acceptable. You can say:
A man I knew nothing about entered the room.
A man about whom I knew nothing entered the room,
The man, about whom I knew nothing, entered the room.
Into the room came the man, about whom I knew nothing.
In the first two of these, the man has not been previously mentioned. In the second two, he has.
The fifth is acceptable, but I would prefer The patient stopped breathing, which caused the doctor to panic.
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