Word Order at the Beginning of the Sentence

As you have seen, in German as in English word order,

the subject (with its modifiers) is usually in the first position,
the verb in the second,
the object in the third,
followed by any prepositional phrase a separable prefix, etc.

One could say that German has a normal or unmarked word order and this word order is:

subject	verb	objects	prepositional phrase
1st position	2nd position	3rd position	(4th position)

"Unmarked" means the speaker puts no special emphasis on any of the elements in the sentence.

Examples:

1st position	2nd position	3rd position	4th position
Anna	öffnet	die Tür.
Ich	studiere	Germanistik.
Er	stellt	das Bücherregal	an die linke Wand.
Die drei Freunde	sitzen	in der Küche.

However, if a speaker decides to start the sentence with something other than the subject, for example with an adverb or an object or a prepositional phrase, the subject has to be placed after the verb. Another way of thinking of this is that the verb always wants to be in second position, and the rest of the sentence must be arranged around that.

This change in word order, which German enforces regularly, is called the inversion of the subject and verb.

English only does this rarely: "Rarely have I used inversion in English!". In fact, we only use inversion if we begin a sentence with 'never' or 'rarely'.

We had a couple of examples for this rule in our dialogue:

1st position	2nd position	3rd position	4th position
Aber eigentlich	arbeite	ich	bei Jochen.
Das Bett	stelle	ich	an die linke Wand.
Erst	gehen	wir	auf den Markt.

When would a German put something else besides the subject in the first position?

Good question, but not that easy to answer!

As soon as s/he intends to give a little more emphasis on one of the elements, s/he may move it out of its unmarked position and to the beginning of the sentence.