①Only by looking back on your past, can you look forward to the promising future.
②Only when you look back on your past, can you look forward to the promising future.
①only you look...，can you ...：没有when相当于出现了两个谓语
Being a man has always been dangerous. There are about 105 males born for every 100 females, but this ratio drops to near balance at the age of maturity, and among 70-year-olds there are twice as many women as men. But the great universal of male mortality is being changed. Now, boy babies survive almost as well as girls do. This means that, for the first time, there will be an excess of boys in those crucial years when they are searching for a mate.