Page 100 - Practical English Usage 3ed - Michael Swan, Oxford

As-, when- and while-clauses can go at the beginning or end of sentences, but as-clauses usually introduce less important information, and most often go at the beginning.
A progressive tense is usually used for the longer 'background' action or situation (was walking; are having; were playing). But as and while can be used with a simple tense, especially with a 'state' verb like sit, lie, or grow.
As I sat reading the paper, tile door burst open.
2 simultaneous long actions: while; as
We usually use while to say that two longer actions or situations go/went on at the same time. We can use progressive or simple tenses.
While you were reading the paper, I was working. John cooked supper while I watched Tv.
As is used (with simple tenses) to talk about two situations which develop or change together.
As I get older I get more optimistic.
We prefer when to refer to ages and periods of life.
When I was a child we lived in London. (NOT AsA'fhile I was a child ...) His parents died when he was twelve. (NOT ••• ftlhile he ftltffl fflJellH!.)
3 simultaneous short actions: (just) as; (just) when
We usually use (just) as to say that two short actions or events happen/ happened at the same time.
As I opened my eyes I heard a strange voice. Mary always arrives just as I start work.
ailst) when is also possible.
I thought of it just when you opened your mouth.
4 reduced clauses with when and while
It is often possible to leave out subject + be after when (especially when it means 'whenever'), and after while. This is rather formal.
Don't forget to signal when turning right.
(= . . . when you are turning right.) Climb when ready.
(= ... when you are ready.)
While in Germany, he got to know a family ofmusicians.
(= While he was ...)
Note that as is usually pronounced /132/ (see 616).
For other uses of as. when and while, see the Index.
74 as if and as though; like 1 meaning
As ifand as though are both used to say what a situation seems like. They can refer to something that we think may be true.
It looks as iftthough it's going to rain.
It sounds as iftthough John's going to cJzange his job.
as if and as though; like 74
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