Name the topic, then talk about it
In conversation you often name the thing you want to talk about before the sentence starts. German then picks it up again with a pronoun in first position. This is called Linksversetzung (left dislocation): the phrase moves out of the sentence to the left, and a pronoun keeps its place.
The same statement, plain and dislocated
Den Film habe ich schon gesehen.
I've already seen that film.
Plain fronting. Fine in speech and writing.
Den Film, den habe ich schon gesehen.
That film, I've already seen it.
Dislocated. The topic gets its own moment first.
Case: matching or loose
In a true Linksversetzung the topic takes the same case as the pronoun that picks it up. The pronoun shows what role the thing plays in the sentence: subject, object or Dativ.
Der Peter, der kommt später.
Peter, he's coming later.
Subject: Nominativ.
Den neuen Kollegen, den kenne ich noch gar nicht.
The new colleague, I don't know him at all yet.
Object: Akkusativ.
Meiner Schwester, der habe ich nichts davon erzählt.
My sister, I didn't tell her anything about it.
Dativ object: the topic is Dativ too.
In relaxed speech the topic often stays in the Nominativ, whatever the pronoun does. This is the freies Thema (hanging topic): the topic is just named, without being fitted into the sentence. It can also be picked up by da, dort or a personal pronoun, or not at all.
Der Chef, mit dem rede ich heute nicht mehr.
The boss, I'm not talking to him again today.
Nominativ topic, Dativ pronoun after mit.
Urlaub, da hatten wir dieses Jahr echt Pech.
Holidays, we were really unlucky this year.
Picked up by da.
Die Kinder, die sind jetzt alle aus dem Haus.
The kids, they've all moved out now.
When to use it
Linksversetzung is a speech pattern. It helps when you open a new topic, return to an old one or set up a contrast. In formal writing, use plain fronting or the topic phrases was … angeht and was … betrifft (as for, as far as … is concerned).
Same topic, two registers
Speech
Das Geld, das kriegst du nächste Woche.
The money, you'll get it next week.
Den Wein, den fand ich gut. Das Essen, das war eher nichts.
The wine, I liked it. The food, not so much.
Writing
Was die Rückzahlung angeht, erhalten Sie den Betrag nächste Woche.
As for the refund, you will receive the amount next week.
Den Wein fanden wir gut, das Essen weniger.
We liked the wine, the food less so.
