I cycled to school. My phone slipped out of my pocket. (while)
My phone slipped out of my pocket while I was cycling to school
Some people in the street made a lot of noise last night. I tried to study (while)
Some people in the street were making a lot of noise last night while I was trying to study
Lee and Karen knew each other for several years. They became close friends. (before)
They had known each other for several years before they became close friends.