As vs. When vs. While

When and while

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

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

• I was having a bath when the telephone rang. OR When the telephone rang I was having a bath.
• While they were having a nap, somebody broke into the house.
• I was working on that report when Sam called.
• She was waiting for a bus when the accident occurred.
• Cathy broke her arm while she was playing in the garden.

When can mean at the same time as something else.

• I always wear gloves when I wash clothes.
• She always takes her cellphone when she goes out.

When and while: differences

When is used to refer to ages and periods of life. As and while are not possible in this.

• His parents died when he was ten.
• I was eighteen when I got my driving license.
• She was twenty-one when she got married.

While

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.
• You can do the dishes while she cooks dinner.
• While she was in Australia, she met a rich businessman and fell in love with him.
• I acquired a strong Canadian accent, while I was holidaying in Toronto.
• I sprained my ankle while I was playing football.

When and while in reduced clauses

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

• Start when ready. (= Start when you are ready.)
• While in Australia, we saw many kangaroos. (= While we were in Australia, we saw many kangaroos.)
• I hurt my back, while lifting that box. (= I hurt my back, while I was lifting that box.)
• While in Rome, do as Romans do. (= While you are in Rome, do as Romans do.)
• I learned French, while working in France. (= I learned French, while I was working in France.)

As

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. OR When I opened my eyes, I saw a strange sight.

As can also be used to talk about two situations that develop 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.

As can be used to introduce a longer background action or situation.

• As I was driving down the street, I saw Peter.