_______________was walking down the street，I saw some policemen． _______they heard the news，they were surprised． ______the child was playing with his toys，his parents were watching TV. They shouted slogans ___they passed the square.

when表示从句动作和主句动作同时发生，或先于主句动作； while表示从句的动作与主句动作同时发生，并有延续的含义； as从句动作与主句动作同时发生，常译为“一边…一边…” 。 从句动作和主句动作同时发生，且从句动作为延续性动词时， when，while，as都可使用.

When/While/As

When

While

as

二、when／while／as的用法。