Wenn überhaupt, versuche ich nur einen Boulder.

Wenn überhaupt is a fixed expression meaning if at all. It adds a cautious, limiting tone.

So here it suggests something like:

• I may not try any boulder at all.
• But if I do, it will be only one.

It is not a full literal when-clause here. It works idiomatically as a short comment on the whole statement.

Yes, wenn often means when or if, depending on context. But in wenn überhaupt, it is part of an idiomatic phrase.

So although wenn by itself can mean:

• when in repeated or general situations
• if in conditional statements

in wenn überhaupt, the whole expression means if at all. It should be learned as a set phrase rather than translated word by word.

Because Wenn überhaupt is functioning like an introductory phrase before the main clause:

• Wenn überhaupt, versuche ich nur einen Boulder.

The comma separates that introductory element from the main clause.

German often uses commas after fronted adverbial phrases or short clause-like expressions, especially when they introduce the speaker’s attitude or a condition.

Because German uses the verb-second rule in main clauses.

The first position is taken by Wenn überhaupt. Once that happens, the finite verb must come next:

• Wenn überhaupt, versuche ich ...

Structure:

• Position 1: Wenn überhaupt
• Position 2: versuche
• then: ich

If you started with ich, then you would say:

• Ich versuche, wenn überhaupt, nur einen Boulder.

That is also possible, but the original sentence puts stronger emphasis on Wenn überhaupt.

In this sentence, überhaupt strengthens the idea of at all.

It often appears in phrases like:

• wenn überhaupt = if at all
• gar nicht überhaupt is not standard, but überhaupt nicht = not at all
• Hast du überhaupt Zeit? = Do you even have time at all?

Here, it emphasizes doubt or limitation: maybe not even one.

Here, nur limits einen Boulder.

So the idea is:

• not several boulders
• just one boulder

In other words, nur means only and applies to the amount/number.

Compare:

• Ich versuche nur einen Boulder. = I only try one boulder.
• Ich versuche einen Boulder nur heute. = I try one boulder only today.
  Here nur would limit heute, not einen Boulder.

So placement matters.

Because Boulder is masculine: der Boulder.

It is the direct object of versuchen, so it takes the accusative case:

• nominative: ein Boulder
• accusative: einen Boulder

That is why the sentence has:

• Ich versuche einen Boulder.

Not exactly in everyday use.

In climbing German, der Boulder usually means a bouldering problem or short climbing route/problem, not just a large rock.

So in a climbing context:

• einen Boulder versuchen means to attempt a boulder problem

Outside climbing, English speakers might think of a literal big stone, but in German sports language this word is very common in the climbing sense.

Versuchen can be used in two common ways:

1. with a noun object
   
   • Ich versuche einen Boulder.
2. with zu
   • infinitive
     
     • Ich versuche, einen Boulder zu klettern.

Both are possible, but they mean slightly different things in form:

• einen Boulder versuchen is a shorter, very natural climbing-style expression
• versuchen, einen Boulder zu klettern is more explicit

So nothing is missing here. Einen Boulder is simply the direct object.

Grammatically, it is present tense:

• versuche = present tense, first person singular

But in German, the present tense is often used for future meaning when the context makes it clear. So depending on context, this could mean:

• If at all, I’m only trying one boulder.
• If at all, I’ll only try one boulder.

German does this very often and does not always need a separate future form.

Usually wenn überhaupt is the standard idiomatic phrase.

Falls überhaupt is possible in some contexts, but it is much less common and does not sound as fixed or natural here. For a learner, the safest choice is:

• wenn überhaupt

It is best learned as one chunk.

That is possible, but it changes the rhythm and emphasis.

• Wenn überhaupt, versuche ich nur einen Boulder.
  This strongly foregrounds the limitation right away.
• Ich versuche nur einen Boulder, wenn überhaupt.
  This sounds more like an afterthought added at the end.

So the original sentence is especially natural if the speaker wants to start with the cautious condition.