As, When and While

As, when and while can be used to talk about actions that take place at the same time. There are some differences.

Background actions
All three words can be used to talk about a longer background action that was going on when something else happened.

While / as / when they were sleeping, somebody broke into the house.
As / when / while I was walking down the street, I saw a strange sight.

Note that as-, when- and while- clauses can begin a sentence. They can also be put at the end of sentences.

We normally use progressive tenses in clauses beginning with as, when or while. Simple tenses are also possible especially with verbs like sit, lie or grow.

To talk about simultaneous long actions, we usually use while. Both continuous and simple tense forms can be used.

While you were playing I was working.
John tidied the room while Jane cooked supper.

As can be used with simple tenses to talk about situations that change together.

As I get older, I get happier.

When can be used to talk about ages.

When we were children we lived in a farm house.

The expressions just as or just when are used to talk about two short actions that happen at the same time.

Just as I opened my eyes, I saw a strange sight.
Just when I stepped out, the telephone rang.

After when and while, it is possible to leave out subject + be.

Start when ready. (= Start when you are ready.)
While in Rome, do as Romans do. (= While you are in Rome, do as Romans do.)