(1)which用来指物（此时不用that）
The BBC World Service broadcasts all kinds of programs, which change from month to month.
(2)who用来指人
Charlie Chaplin, who died in 1977, was one of the world’s greatest actors.
(3)whom在句中作宾语
The doctor, whom the nurse is talking to, is leaving for Africa next week.
(4)when在句中作时间状语
He went to the States in 1912, when he was noticed by an important film director.
