As, while and when are words we can use to introduce “background” action or situation, which is/was going on when something else happens/happened.For example:
a. As I was running in the park, I saw John chasing his dog.
b. While I was washing the dishes, somebody stole the car.
c. The dog always barks when you enter the door.
As, when and while clauses can go at the beginning or end of sentences, but as clauses usually introduces less important information, and most often go at the beginning.A progressive tense is usually used for the longer “background” action or situation. But as and while can be used with a simple tense, especially with a “state” verb, like sit lie or grow.
a. As I lie watching TV, the window popped open.
Usually, we used while to say that two longer actions or situations go/went on at the same time. We can use progressive or simple tenses.For example:
a. While you were dancing in the bar, I was drinking beer.
b. Dana scratched my back while I read a book.
As is used to talk about two situations which develop or change together.For example:
a. As I get older, I get wiser.
We prefer when to refer to ages and periods of life.For example:
a. When I was young I ate bananas.