In German, the comma is not an optional mark reflecting the writer's tone or reading rhythm, the way it often functions in many other languages. It is a purely grammatical element governed by a strict, finite set of rules, and every correctly written German sentence has exactly one valid comma placement — there is no room for personal judgment. This fundamental difference is why Kommasetzung is taught and tested as its own topic at C1: examiners treat correct comma placement as a marker of linguistic maturity on par with syntax itself.
The central rule governing most cases is that every subordinate clause (Nebensatz) is separated from the main clause (Hauptsatz) by an obligatory comma, regardless of whether it comes before, after, or inside the main clause — as in Ich weiß, dass es schwer ist, or Mein Onkel, der in Berlin wohnt, kommt morgen, where the embedded relative clause is enclosed by two commas. This single rule covers the great majority of real cases, since any subordinating connector such as dass, weil, or obwohl, or any relative pronoun, automatically triggers a comma before it.
Beyond that, several narrower but frequent rules apply: in lists, items are separated by commas except the final one introduced by und or oder, which takes none (Brot, Milch und Käse, not Brot, Milch, und Käse); a comma always precedes aber and sondern, since both are treated as connecting two independent clauses even when the second is elliptical; and an explanatory apposition that further defines a preceding noun, as in Mein Vater, ein Lehrer, lebt in Casablanca, is enclosed by commas on both sides exactly like an embedded clause.
The one case that leaves the writer genuine freedom is joining two complete main clauses with und, where both Ich gehe und du bleibst without a comma and Ich gehe, und du bleibst with an optional comma (added for rhythm or to distinguish different subjects) are correct. In every other case the rule is mandatory, not a matter of taste.
The most frequent error among advanced learners, often coming from backgrounds where comma use is far more flexible, is forgetting the second, closing comma of a subordinate or interposed clause that sits mid-sentence rather than at the end — writing Mein Onkel, der in Berlin wohnt kommt morgen with only one comma, when the rule requires the embedded clause to be bracketed on both sides. In C1 writing exams, especially DSH and TestDaF, concrete points are deducted for every missing or superfluous comma, which makes a final comma check an indispensable last step before submitting any text.