As, when or while?

We can use as to introduce two events happening at the same time. After as, we can use a simple or continuous form of the verb. The continous form emphasises an action that interrupts or occurs during the progress of another action:

As she walked to the door, she thanked them for a lovely dinner.

As they were signing the contract, they noticed that a page was missing.

We can use when to introduce a single completed event that takes place in the middle of a longer activity or event. In these cases, we usually use a continuous verb in the main clause to describe the background event:

He was walking back to his flat when he heard an explosion.

Depending on the context, when can mean ‘after’ or ‘at the same time’.

We often use just with when or as to express things happening at exactly the same time:

The phone always rings just when I’m closing the front door.

She was a brilliant gymnast, but she had a terrible accident in 1999, just as her career was taking off.

We can use while or as to talk about two longer events or activities happening at the same time. We can use either simple or continuous verb forms:

We spent long evenings talking in my sitting-room while he played the music he had chosen and explained his ideas.

We were lying on the beach sunbathing as they were playing volleyball.

We can use when and while without a verb, or without a subject + auxiliary verb be:

Go past the village signpost and you get to a church. When there, take the next turning right. (formal)

He read his book while waiting for the bus. (while he was waiting)