When I write a clause that begins with a clause-containing adjunct, I generally put a comma after the adjunct. The comma in that first sentence illustrates my practice. Some writers studiously avoid such a comma (sometimes my style is known as "heavy" punctuation and the other style as "light"). I also like the so-called "Oxford comma": I write Oregon and Washington, but I don't write California, Oregon and Washington. I use an extra comma and write California, Oregon, and Washington.
I couldn't wish for a better illustration of why I like my own policies than the following sentence, which I saw in The Economist last week (April 4, p. 11). It goes the other way on both of my policies, and it's disastrously misunderstandable in my opinion:
Traders and fund managers got huge rewards for speculating with other people's money, but when they failed the parent company, the client and ultimately the taxpayer had to pay the bill.
Let's strip it down to the bare essentials for discussion, reducing the part I'm concerned with to this:
The problem is that I found myself reading the parent company as the direct object of failed (wrong; what was intended was that I should read failed as intransitive), and then that left the client and the taxpayer to be the subject of had to pay (wrong again; the intention was to have the parent company, the client and the taxpayer in that role).
You don't get the same effect in the following sentence of the article, despite its initial when adjunct:
Monetary policy contributed to this asymmetry of risk: when markets faltered central banks usually rescued them by cutting interest rates.
The reason is twofold. First, falter is a purely intransitive verb: there is no possibility of reading *Markets faltered central banks as if it were a clause. And second, since there is no tripartite coordination, we are forced to read central banks as the subject of usually rescued them because there is no other noun phrase to fill that role. So in that case it doesn't matter whether you follow my policies or not: nothing untoward happens. By contrast, look at this (invented) example:
A comprehension catastrophe, if what you meant was that you were in college, but the army wanted to recruit you and your parents wanted you to go serve your country. In that case you really needed to write this about your abandoning college:
Here's how I would have punctuated the original Economist example, using both the post-adjunct comma and the Oxford comma:
Traders and fund managers got huge rewards for speculating with other people's money, but when they failed, the parent company, the client, and ultimately the taxpayer had to pay the bill.
My policy is better. It's not always vitally necessary, and I'm not rigid about it, but often it is clearly better. It truly does forestall misunderstandings. This is not like one of those dopey rules such as that you mustn't use since to mean "in view of the fact that" because it might lead to ambiguity (the publication manual of the American Psychological Association insists on that one). Since has been used to mean "in view of the fact that" for hundreds of years and I've never seen it lead to a confusing ambiguity. The maintainers of the rule are unjustified both in terms of what's found in good writing and in terms of what's ambiguous and what isn't.
But there have been real people who used the post-adjunct comma and the Oxford comma in their writings (there is more than one convention in play here, I'm not inventing a new one), and they were right to do so, because it really does hold misunderstandings at bay. So follow me, not The Economist.
And don't ever say you never saw a prescriptive statement on Language Log. We aren't opposed to prescriptive statements (though lots of people think we are). We're opposed to stupid prescriptive statements that don't have any point and incorrect prescriptive statements that don't agree with the usage they are trying to prescribe. We're emphatically in favor of obeying widely accepted principles of grammar that genuinely help clear expression and easy comprehension. What's not to like about clear expression and easy comprehension?