Something was going on WHEN another thing happened.
Something was going on WHILE another thing was going on.

I (wait) for a friend, when the alarm (go) off.
It (rain), when they (come) home.
While she (visit) friends he (watch) a football match.
Jane (read) a magazine when the doorbell (ring).
They (paint) the walls while we (clean) the car.
She (do) her homework when her brother (come) to ask a question.
Yesterday, I (walk) to the train station when I (meet) a friend.
While they (do) the housework they (listen) to country music.
It (snow) when they (cause) an accident.
They (cook) dinner while their guests (discuss) politics.
He (sleep) when the plane (land).
They (travel) Europe when the thieves (break) into their house.