In English the topicalisation of declarative clauses is facultative, the subject is in first position, and there may be an additional item in front of it. While in German, declarative clauses are always topicalized.

Die Blätter fallen im Herbst von den Bäumen.

The leaves fall from the trees in autumn.

Im Herbst fallen die Blätter von den Bäumen.

In autumn the leaves fall from the trees.

The main difference is German insists of having exactly one item on front of the finite verb. That's why the subject must follow the finite verb if it isn't the topic.

And this explains how German speakers decide which item goes in front of the finite verb.

It's the topic.

In English the topicalisation of declarative clauses is facultative, the subject is in first position, and there may be an additional item in front of it. While in German, declarative clauses are always topicalized.

Die Blätter fallen im Herbst von den Bäumen.

The leaves fall from the trees in autumn.

Im Herbst fallen die Blätter von den Bäumen.

In autumn the leaves fall from the trees.

The main difference is German insists on having exactly one item on front of the finite verb. That's why the subject must follow the finite verb if it isn't the topic.

And this explains how German speakers decide which item goes in front of the finite verb.

It's the topic.