Der Boulder endet am Ausstieg.

Not necessarily. In climbing German, der Boulder often means a bouldering problem / bouldering route, not just a physical rock.

So in this sentence, Der Boulder most likely means the boulder problem.

In everyday German, Boulder can also refer to a large rock, but in a climbing context, learners should usually think of a climb rather than a stone.

Boulder is masculine in German, so its nominative singular article is der:

• der Boulder = the boulder / the boulder problem

That is why the sentence begins with Der Boulder.

Endet comes from the verb enden, which means to end.

Here it is in the 3rd person singular, present tense, because the subject is der Boulder:

• ich ende
• du endest
• er/sie/es endet

So:

• Der Boulder endet ... = The boulder problem ends ...

Am is a contraction of an dem.

• an dem → am

So:

• am Ausstieg = an dem Ausstieg

In English, this often translates as at the exit/top-out or at the finishing move, depending on the climbing context.

Because am Ausstieg is a prepositional phrase with an.

The preposition an can take either:

• accusative for direction/movement toward something
• dative for location/position

Here, the sentence describes where the boulder ends, not movement toward something, so German uses the dative:

• der Ausstieg → dative: dem Ausstieg
• an dem Ausstieg → am Ausstieg

So the logic is:

• Der Boulder endet am Ausstieg.
• The route ends at the top-out/exit.

In climbing, Ausstieg usually refers to the exit, top-out, or the final section where you get off the boulder.

It is more specific than the everyday English word exit might suggest. In this context, it usually means the point where the climber finishes the problem by getting onto or over the top.

So am Ausstieg is something like:

• at the top-out
• at the exit move
• at the finishing section

This is standard German main-clause word order:

• Der Boulder = subject
• endet = verb
• am Ausstieg = prepositional phrase

German main clauses normally place the finite verb in second position. So this sentence follows the basic pattern:

• Subject + verb + rest

That makes it a very straightforward sentence structurally.

Sometimes, but endet is better here.

• enden = to end
• aufhören = to stop

In a climbing or route description, enden sounds more natural and precise for saying where a route or problem finishes.

So:

• Der Boulder endet am Ausstieg. = natural
• Der Boulder hört am Ausstieg auf. = understandable, but usually less standard in this kind of description

No. The subject is Der Boulder.

You can tell because:

• Der Boulder is in the nominative
• the verb endet agrees with it
• am Ausstieg is just a prepositional phrase telling you where the ending happens

So the structure is:

• Der Boulder = who/what is ending
• endet = ends
• am Ausstieg = at the top-out / exit

A useful rule is:

With two-way prepositions like an, ask whether the sentence expresses:

• movement toward a destination → accusative
• location / position / where something is → dative

Here, endet am Ausstieg describes the location of the endpoint, so it uses dative.

Compare:

• Der Boulder endet am Ausstieg.
  The problem ends at the top-out. → dative

If it were about movement toward something, you would expect accusative instead.