When, while, as - used to express time
when is used, with simple tenses:
1 When one action occurs at the same time as another or in the span of another:
- When it is wet the buses are crowded.
- When we lived in town we often went to the theatre.
2 When one action follows another:
- When he explained the situation she calmed down.
as is used:
3 When the second action occurs before the first is finished:
- As I walked through the park I remembered about my mother's birthday.
The above sentence implies that I remembered about my mother's birthday before had completed the action of walking thorugh the park; I was probably still in the park.
While I was walking would have the same meaning here, but When I walked would give the impression that the act of walking through the park was complete and he was no longer in the park.
4 For parallel actions:
- The men whistled as she walked past.
5 For parallel development:
- As he listened to the story he grew more and more curious.
- The darker it grew, the colder it became.
- As he worked harder he was getting more and more money for his work.
If we used when here we would lose all idea of simultaneous progression or development.
6 To mean while:
- As he was watching TV his wife was preparing a meal.
However, there is no particular advantage in using as here, and while is safer.