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.**