There are two basic sentence types in German: main clauses, which are also know as independent clauses and subordinate clauses, which are also known as dependent clauses. Main or independent clauses can stand alone, whereas subordinate or dependent clauses require a sentence to which they are linked.
Unlike in English, German main clauses and subordinate clauses are separated more frequently with commas. For example, this is true for sentences that are connected with the conjunction “dass” (that).
1. Ich weiß, dass er morgen Geburtstag hat. – I know that it is his birthday tomorrow.
2. Er ist verärgert, dass sie sich nicht bei ihm gemeldet hat. – He is annoyed that she hasn’t got in touch with him.
3. Ich glaube nicht, dass das funktionieren wird. – I don’t think that it will work.
4. Ich bin traurig, dass du meinen Geburtstag vergessen hast. – I’m sad that you have forgotten my birthday.
When you have a look at the English sentences you may have recognized that there are two main clauses in each sentence. In contrast, the German sentences consist of a main clause AND a subordinate clause each. Why is that?
You may already know that German word order is fairly variable. German main clauses follow the rule: subject + predicate + object. But German subordinate clauses do never follow that rule. Instead, subordinate clauses in German usually undergo a kind of modification, that is, you have to relocate the respective words. This also happened to the words in the sentences above. The conjunction “dass” (that) introduces a subordinate clause, which requires a rearranging of the S-P-O word order. If you don’t do that, your statement would hardly make any sense and your subordinate clause would no longer be a subordinate clause but a main clause.
Here is how the subordinate clauses from above would look like when we rearrange the word order into S-P-O, that is, main clauses.
5. …, (dass) er morgen Geburstag hat: Er hat morgen Geburstag. – It is his birthday tomorrow.
6. …, (dass) sie sich nicht bei ihm gemeldet hat: Sie hat sich nicht bei ihm gemeldet. – She hasn’t got in touch with him.
7. …, (dass) das funktionieren wird.: Das wird funktionieren. – It will work.
8. …, (dass) du meinen Geburtstag vergessen hast.: Du hast meinen Geburtstag vergessen. – You have forgotten my birthday.