There's a rumour going around about clauses and commas. For example,look at the KS3 Framework for English, Year 7, sentence-level. What does it say about clauses in complex sentences? Pupils should be able to use commas well as "boundary signposts". If you've tried to teach that, you'll know why we think it's misleading.
Okay, very often a clause does need a comma to mark it off, but that's not because it's a clause. English isn't German. In German, there's a very simple rule: punctuate every subordinate clause boundary with a comma (or full stop, of course). So German schoolchildren learn to write like this:
"We know, that you wonder, where this is all leading." (Except that they use German, of course.) But that's terrible English. Heaven forbid that any of our children would use commas like that - and yet that's what the rumours seem to recommend.
The fact is that English commas have nothing whatever to do with clauses.
If a clause gets a comma, it's for some other reason - not just because it's a clause. Heresy, you say? Well, just consider one of the most popular comma rules: the one for sentence-initial subordinate clauses.
This says: If a subordinate clause stands at the start of a sentence, put a comma after it (as in this sentence, in fact). True, this often gives the right result, but it actually misses the target badly by both overshooting and undershooting at the same time.
lIt overshoots by aiming at too general a target: "subordinate clauses at the start of a sentence". This leads to undesirable commas like the one in: "What I bought, cost me five pounds" or "That smoking kills people, is an established fact." It should in fact be limited to just one of the three main kinds of subordinate clause: adverbial clauses. Adverbial clauses are the ones that are used in the way that adverbs ("actually") or prepositional phrases ("in recent years") are used.
lIt undershoots by aiming at too specific a target - clauses - when it actually applies to any adverbial, be it a phrase or clause. For example, here's a paraphrase of the earlier sentence: "In the case of a subordinate clause at the start of a sentence, put a comma after it." We've replaced the subordinate clause by a phrase ("in the case of ..."), but we still need the comma. So: At the start of a sentence, an adverbial needs a comma to separate it from the rest of the sentence. (Incidentally, that sentence is an example of the rule, and so is this one.) Forget clauses - think adverbials. But of course the new rule only makes sense to those who know what an adverbial is. If you're not sure about that, come back next week.