This is actually very easy, but people often get it wrong.
Most people whack in a comma at the end of all opening clauses that contain a verb. In Australia, at least, this is not the right way to go about things. Sometimes such clauses take a comma; sometimes they don’t.
If you’re Australian and you want your editing to be above reproach, read on. It isn’t hard to get your head around. There’s actually a simple rule you can follow. It is:
Clauses introduced by as. since and while that express time, don’t take a comma. When these same clauses express a cause, they do.
While he was waiting for her to get ready he drank three martinis. (Time)
While you might think otherwise, I’m convinced I’m right. (Cause)
As he was thinking these thoughts a butterfly flew past the window. (Time)
As you no longer wish to go to the moves with me, I will go alone. (Cause)
Since World War One ended society hasn’t been the same. (Time)
Since you no longer wish to be president, I nominate Susan. (Cause)
There’s only one thing you need to watch out for. Sometimes clauses introduced by as, since or while and involving time can be confusing if a comma isn’t used.
Since the new boss took over the place hasn’t been the same. (Time)
If you follow the rule slavishly, you won’t use a comma. But without a comma the reader tends to read: Since the new boss took over the place … Round about now they realise the sense is meant to be different from this, and they have to backtrack. To avoid readers backtracking (never a good thing), break the rule and use a comma.
Since the new boss took over, the place hasn’t been the same.
And that’s all there is to it.
Sometimes when is used in the same way as while. For example:
When we were walking in the forest we saw two foxes.
On occasions where when is used in a clause to convey time, the same rules apply.
And there you have it.