when 既可以和延续性动词连用,也可和非延续性动词连用.
When / while / As I was walking in the street ,it began to rain .
I was coking when someone knocked on the door .
While I was doing my homework ,my mother was watching TV .
We sang as we walked along .

1. While I was doing my homework , my mother was watching TV .

2. When  / while / I was walking  in the street , it began to rain .

3. When  he  came in ,  I  was reading the newspaper.

4. When she heard  the sad new, she began to cry .