We use when, as, before, after to show clearly the order in which different events happened. The part of the sentence that begins with the time expression is called the 'time clause'.

when

We use when to refer to actions that happen at almost the same time. One action is an immediate consequence of another. Note that when the time clause comes first, it must be followed by a comma. 
When you turn the handle, the wheel starts to move.

We can change the two parts of the sentence around, but when must always come before the first action in the sequence of events.  
The wheel starts to move when you turn the handle.

When the time clause comes later in the sentence, we do not use a comma to separate the two clauses. 

as 

We use as to talk about two actions that happen at the same time. The position of the time clause can change, in the same way as for when.   
As the wheel turns, it generates electricity.  
The wheel generates electricity as it turns.

Note that in the second as sentence, we need to use a subject the wheel, because by changing the order of the clauses, it is no longer clear what it refers to. 

before and after 

Before and after also indicate the sequence of events, but there is not necessarily such a close time relationship as with when and as. Before and after simply indicate that one action happened at an unspecified time before another. 

Again, the position of the time clause can change.   
Before you operate the machine, you must read the manual  
You must read the manual before you operate the machine  
Trevor Baylis developed his clockwork radio after he visited Africa.   
After he visited Africa, Trevor Baylis developed his clockwork radio. 

Before and after can be followed by the -ing form instead of subject + verb.
Before operating the machine, you must read the manual.  
After visiting Africa, Trevor Baylis developed his clockwork radio.

Last modified: Monday, 22 April 2024, 9:12 AM