I’m no expert on official punctuation rules, and there’s quite a bit of freedom in English anyway, but here’s one thing that will reduce mistake (and annoyance) rates drastically for some people: Germans, please don’t transfer German Kommaregeln to English! I know you’ve probably put a deal of effort into learning this in high school, but German punctuation rules, I regret to inform you, will lose rather than win you points in English. So please: DON’T PUT A COMMA IN FRONT OF EVERY THAT! That can be a relative pronoun (like which, which requires a comma if/because a non-defining relative clause ensues) or a conjunction, and in neither case does it need a comma!
Personally, I don’t think we should transfer English punctuation practices to German, either. I never put commas in front of the main verb (as in Anders gesagt, sind…)—after all, German’s a V2 language, and the position before the verb can be filled by complex constituents, so there’s no reason why I should. 🙂 But I think the Duden’s against me on this one. 🙁 In any case, you should have a good reason for putting a comma before the main verb in German. This is a general rule of thumb, actually: You should have a good reason for any comma you put.
Here’s a case in point, from a recent paper I’ve received:
In English, unlike in German, not all relative clauses should have commas. A comma before a relative clause indicates that the relative clause is a non-defining relative clause, i.e. a relative clause that can be omitted because it gives extra information that is not essential to the clause. In the example above, the comma suggests that the conversational part of the BNC is the only corpus that was queried for the paper. But that is not, in fact, the case, which is why I was confused when I read the sentence. The paper in fact draws on several corpora. What the author wanted to say is that the BNC is the only one of the corpora used where the speakers were recorded at unawares. To convey this piece of information, the comma should have been left out.