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When, while, or as
When marks the moment something happens, while covers a stretch of time or a contrast, and as means at the same moment or as things change together.
Quick rule
A moment, an age, or a question? When. Something long at the same time, or a contrast? While. The same moment, or things changing together? As.
At the time that something happens, or at what time: when I was nine, when does it start.
During the time that something else happens, or to contrast two things: while I wait, while he likes tea, she likes coffee.
While something else is happening, at the same moment, or as things change together: as she grew older, as the evening went on.
How to choose
| Question | when | while | as |
|---|---|---|---|
| Can it ask a question (___ does it start?)? | Yes | No | No |
| Does it contrast two different things (My sister is tall, ___ I am short)? | No | Yes | No |
| Does it show two changes moving together (___ the city grew, so did the traffic)? | No | No | Yes |
| Does it go with a short event (___ the phone rang)? | Yes | SometimesSometimes | SometimesSometimes |
Why it trips people up
When, while, and as can all describe two things happening at the same time, so they feel interchangeable. The difference is the picture: a moment (when), a stretch of time or a contrast (while), or changes moving together (as).
How each one works
When
An age or period of life
When I was nine, we moved to Quito.
To ask about the time
When does the next train leave?
A future moment, with the present tense
I will send the photos when I get home.
While
Two actions at the same time
I read the news while I wait for the bus.
Two long actions in parallel
While Ana set the table, I made the salad.
Contrast: on the other hand
My brother loves winter, while I prefer summer.
As
At the same time
As we drove home, it started to snow.
Two changes moving together
As the evening went on, the music got louder.
At that very moment
She smiled as she opened the gift.
What comes next
when plus clause (a moment)
The kids went quiet when the teacher walked in.
when plus do/does plus subject (question)
When do you start your new job?
while plus subject plus was/were plus -ing
While I was washing dishes, the power went off.
clause, while plus clause (contrast)
Rice is cheap here, while beef is expensive.
as plus clause (at the same time)
The bus left as I reached the stop.
as plus change, so plus change
As the weather warms up, the beaches fill up.
Set phrases
When
- when it comes to
- used to introduce a subject you want to talk about
While
- once in a while
- sometimes, but not often
- while you're at it
- used to ask someone to do one more thing during the same job
As
- as and when
- at some future time, but only after something else happens
- as for
- used to start talking about a person or thing
Almost the same
When the movie ended, we walked home.
It ended first, then we walked.
While the movie played, we ate popcorn.
Both things happened together.
As the movie ended, the lights came up.
It happened at that exact moment.
When the days got shorter, we stayed in.
It points to when it happened.
As the days got shorter, we stayed in more.
The two changes move together.
In a conversation
- A:
When did you start learning English?
- B:
When I was twelve. I practiced while I watched cartoons.
- A:
Really? I hated it as a kid, while my brother loved it.
- B:
It got easier as I listened to more songs.
Common mistakes
Wrong: While walking to school, the rain started.
Right: While I was walking to school, it started to rain.
An -ing clause after while needs the same subject as the main clause. Here it is the rain that "walks", which reads as a dangling modifier.
Wrong: We will start dinner when Dad will come home.
Right: We will start dinner when Dad comes home.
After when, a future meaning uses the present simple, not will.
Memory trick
Swap in "at the moment that" for when, "during the time that" for while, and "gradually as" for as. To ask for the time or date, only when works.
It depends
- More meanings of as
- Besides "at the same time", as can give a reason (As it was late) or mean "in the way that" (do as I showed you). Reason clauses are covered in since, as, or because.
- While as contrast
- While also compares two things and sometimes means although, according to Oxford: While I am willing to help, I have little time.
FAQ
What is the difference between when and while?
When points to a moment, an age, or a question about time. While covers a stretch of time during another action, or a contrast.
What is the difference between as and when?
When names the time something happens. As says two things happen at the same moment, or that two changes move together.
When do I use when, while, or as?
Use when for a moment or a time question, while for a stretch of time or a contrast, and as for things that change together.