Position one is a choice
The slot in front of the conjugated verb, the Vorfeld, can hold the subject, an adverbial or an object. English keeps the subject there and uses stress, it was X that… or as for… to highlight something. German simply puts the phrase first.
One event, four openings
Meine Schwester hat mir das Buch gestern geschenkt.
My sister gave me the book yesterday.
Neutral. Nothing is singled out.
Gestern hat mir meine Schwester das Buch geschenkt.
Yesterday my sister gave me the book.
Das Buch hat mir meine Schwester geschenkt.
The book, my sister gave me.
The book is the topic, the news is who gave it.
Mir hat sie das Buch geschenkt, dir hat sie nichts mitgebracht.
She gave me the book, but she didn't bring you anything.
What the first position does
It links to the previous sentence
Wir haben lange über den Vertrag gesprochen. Danach war allen klar, was zu tun ist.
We talked about the contract for a long time. After that everyone knew what to do.
Sie hat ein neues Fahrrad. Damit fährt sie jeden Tag zur Arbeit.
She's got a new bike. She rides it to work every day.
It sets up a contrast
Kaffee trinke ich gern, Tee überhaupt nicht.
Coffee I like, tea not at all.
Im Sommer ist es hier voll, im Winter kommt kaum jemand.
In summer it's packed here, in winter hardly anyone comes.
It sets the frame: time, place, condition
In Deutschland zahlt man oft noch bar.
In Germany people often still pay cash.
Wenn du willst, hole ich dich am Bahnhof ab.
If you like, I'll pick you up at the station.
A paragraph in which every sentence starts with the subject is correct but reads flat. Vary the opening: a time word, a place, a word from the previous sentence.
Fronting an object
This only works safely while the case marking shows that the phrase is an object. den can only be an Akkusativ object. With feminine and neuter nouns Nominativ and Akkusativ look the same, and German reads the first noun phrase as the subject.
Den Chef hat der Praktikant angerufen.
The intern called the boss.
den marks the object, so the meaning is clear.
Die Chefin hat die Praktikantin angerufen.
The boss called the intern. (It cannot mean the other way round.)
Both are feminine, so nothing marks the object: the first phrase is always read as the subject.
