a. What would the world be like if we had nothing but water to drink?
b. If I knew Judy’s new address, I could send her a birthday card.
cf. As I do not know Judy’s new address, I cannot send her a birthday card.
a. In 1970, when the International Exposition was being held, my parents moved to Osaka.
b. I arrived in Okinawa on May 7, when the rainy season started there.
c. The custom goes back to the late eighteenth century, when she began to have tea and cake around 5 p.m.