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"Do you remember when we went to New York?"
"I do."
"That's when Jack and Mary fell in love."[/quote]

This implies that the question "when did Jack and Mary fall in love" has been asked, but...

Блин, я сейчас с ума сойду. When или then?[/quote]

To me, "it was when Jack and Mary fell in love" sounds odd, like they were walking down the street and bang, they fell in love. "It was then"...

my 2 cents...
It was then that Jack and Mary fell in love. ( while "with each other" literally translates the Russian, it's redundant in English and is never expressed that way.)