Der Fuß ist auf dem Tritt.

Because Fuß is a masculine noun in German. German nouns have grammatical gender, and Fuß happens to be masculine, so in the nominative singular it takes der:

• der Fuß = the foot

In this sentence, der Fuß is the subject, so the nominative form is used.

Because auf can take either the dative or the accusative, depending on the meaning.

Here, auf dem Tritt describes a location: the foot is on the step/tread. That means German uses the dative.

• auf dem Tritt = on the step/tread

If the sentence described movement onto the step, German would use the accusative:

• Der Fuß geht auf den Tritt. = The foot goes onto the step.

A useful rule is:

• location → dative
• direction/movement toward a destination → accusative

It is dative singular.

The noun Tritt is masculine, and the definite article for a masculine noun in the dative singular is dem:

• nominative: der Tritt
• accusative: den Tritt
• dative: dem Tritt

So auf dem Tritt is a dative phrase because it gives a location.

Tritt can have several related meanings depending on context, such as:

• step
• tread
• foothold
• sometimes pedal in certain contexts

In this sentence, it most naturally means something like step or tread, meaning the surface the foot is resting on.

So it is not the action of stepping here, but the physical thing being stood on.

Ist is the 3rd-person singular form of sein = to be.

The sentence is built like this:

• Der Fuß = subject
• ist = verb
• auf dem Tritt = location

So the structure is basically:

• The foot is on the step/tread.

German often uses sein to describe where something is.

Because all nouns are capitalized in German.

So:

• Fuß is a noun
• Tritt is a noun

and both must begin with a capital letter, even in the middle of a sentence. This is a standard rule in German spelling.

In standard German spelling, Fuß is written with ß because the vowel u is long here.

Compare:

• Fuß = foot
• Kuss = kiss

A common spelling rule is:

• long vowel + ß
• short vowel + ss

So Fuß has a long u sound, which is why it is spelled with ß.

Yes, German word order is somewhat flexible, but the conjugated verb usually stays in the second position in a normal main clause.

The basic version is:

• Der Fuß ist auf dem Tritt.

But you could also say:

• Auf dem Tritt ist der Fuß.

That version is grammatically possible, but it sounds more marked or stylistically unusual in many contexts. The normal, neutral order is the original one.

Possibly, depending on context, but it changes the nuance a little.

• Der Fuß ist auf dem Tritt simply states where the foot is.
• Der Fuß steht auf dem Tritt suggests more specifically that the foot is placed/stationed/standing on it.

With body parts, ist is often the safer and more neutral choice when you are just describing location.

Yes, the definite article der normally suggests a specific foot: the foot.

In German, body parts are often used with the definite article where English might sometimes use a possessive:

• Er hob die Hand. = He raised his hand.
• Der Fuß ist auf dem Tritt. = The foot is on the step/tread.

So using der here is completely natural in German.