What is the difference between "Zu" and "Auf" in these sentences?

Question

What is the difference between

Ich blickte zu Boden

and

Ich blickte auf Boden

?

Answer 1

Zu Boden simply means that your head was was tilted towards the floor, e.g. due to shyness. Auf den Boden (the article can't be omitted here) would be used for looking at the carpet, searching for something you have accidentally dropped or spilled on it, or anything placed there.

Answer 2

Well, the most conspicuous difference is, that one sentence is correct and the other is wrong:

correct: Ich blickte zu Boden.
wrong: Ich blickte auf Boden.

But you can turn the second sentence into a correct sentence by adding an article:

correct: Ich blickte auf den Boden.

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zu Boden

The fixed phrase »zu Boden« just means a movement and the direction of this movement is downwards (»down to the ground«):

Bei der Flucht wurden einige Menschen zu Boden getrampelt.
^{During the escape, some people were trampled to the ground.}

Werft ihn zu Boden!
^{Throw him to the floor!}

Nach einem hefigen Treffer ging der Boxer zu Boden.
^{After a fierce hit the boxer kissed the dust. (literal: ... the boxer went to the ground.)}

But you can also use this phrase to describe a movement of somebodies eyes:

Sie blickte verschämt zu Boden.
^{Bashfully she lowered her gaze. (Literal: She looked bashfully to the ground.)}

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auf den Boden

There is no similar fixed Phrase »auf Boden«. But you still can use the preposition »auf« together with the noun »Boden«, but if you do so, you also have to use an article between both words. And since the preposition »zu« wants dative case, and because there is only one ground (i.e. singular), and because »Boden« is a masculine noun, the correct article is »den«: »auf den Boden«.

Now the meaning is a movement whose target is the ground. This is a very subtile difference to the movement described before which was a movement in a direction. Now it's a movement that ends at a place. In German this is not the same, and you can see that also in English there is a difference:

zu Boden = movement to the ground
auf den Boden = movement that ends on the ground

Here are examples:

Irene legte sich auf den Boden.
^{Irene lay down on the ground.}

Er warf sich voller Demut vor dem König auf den Boden.
^{He threw himself on the ground before the king with humility.}

Der Küchenjunge ließ ein Ei auf den Boden fallen.
^{The kitchen boy dropped an egg on the floor.}

In the context of looking somewhere, you can also look at the ground:

Der Inspektor blickte nur kurz auf den Boden und sah sofort die Fußspuren.
^{The inspector looked at the ground only for a moment and immediately saw the footprints.}