Which of the following is acceptable? #1 World War II was over in 1945 when my mother was born. #2 World War II was over in 1945, when my mother was born. #3 World War II was over in the year when my mother was born.
I think #2 and 3 are acceptable, but I’m not sure about #1.
Could you also explain why some of these are acceptable and others not?
#1 is not acceptable because ‘1945’ is already definite; it should not be followed therefore by a defining clause. The others are OK, except that ‘was over’ is the wrong verb choice, since the war was not over in January-July 1945. Replace ‘WW II was over’ with ‘WW II ended’.