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As, when and while

All three of these words can be used to talk about actions or situations that take place at the same time. There are some differences.

Background action or situation

We can use all three words to introduce a longer background action or situation, which is/was going on when something else happens/happened. Note that we usually use a progressive tense for the longer background action or situation.

As-clauses usually introduce less important information. They most often go at the beginning.

• As I was driving to work, I saw him walking down the street.
  

When- and while-clauses can go at the beginning or end of sentences.

• I was having a bath when the telephone rang.
• While they were having a nap, somebody broke into the house.

Simultaneous long events

While is used to say that two longer actions or situations go/went on at the same time. We can use progressive or simple tenses.

• While he was watching TV, I was working.
  
• He slept while I cooked supper.
  

As is used to refer to two situations which develop or change together. We normally use simple tenses.

• As I get older, I get more optimistic.
  
• As he came into the room, all rose to their feet.
  

When is used to refer to ages and periods of life.

• His parents died when he was ten.
  

Simultaneous short events

To talk about two short actions or situations that happen/happened at the same time, we usually use as. When is also possible.

• As I opened my eyes, I saw a strange sight.
  

Reduced clauses

It is often possible to drop subject + be after when and while.

• Start when ready. (= Start when you are ready.)
  

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