Every time we write, translate or correct a text, we use punctuation marks. The main idea of punctuation is to divide the text into information blocks, which at the same time serves to make the text easier to read and thereby understand.
Punctuation has always been a tricky part of learning a language, e.g. some languages use the grammatical comma, where the comma divides the main and the subordinate clause. In other languages there is no comma between the main and the subordinate clause. However, you can also find languages, which doesn’t use punctuation at all. This is seen in the Thai language, where the words are written in one continual line or in the rune language, where a space was used to indicate a new sentence.
Where to use what?
The English punctuation rules
If you want to get a deeper knowledge of the English punctuation and grammar rules you can have a look at this homepage. Here you get all the rules and there is even a test that you can take at the end of each topic.
The Danish punctuation rules
Back in 2004 Denmark changed its traditional grammatical punctuation (like the one we know from e.g. German) to a more flexible punctuation rule (like the English, Swedish and Norwegian). The Danish punctuation rules are set by Danske Sprognævn (the Danish language council) and can be found in Retskrivningsordbogen (the Danish spelling dictionary).
Where the old comma divided the main and the subordinate clauses, the new gives the writer an option to leave out the comma between the main and subordinate clauses. Because the rules are many I suggest that you have a look in the newest edition of Retskrivningsordbogen. You can also get an overview here.
If you want to test your Danish comma skills you can do so here.