Learn Hardcore German: Der Bergpfad ist schmal, aber sicher, wenn man langsam geht. - The mountain path is narrow but safe if you walk slowly.

Pfad → masculine → der Pfad, so also der Bergpfad.
The first part of the compound (Berg) does not affect the gender; the last part (Pfad) decides it.
So you must learn the gender with the base noun: der Pfad, die Straße, das Tal, etc.

What exactly is the structure and meaning of the compound noun Bergpfad?

Bergpfad is a compound of:

Berg = mountain
Pfad = path, trail

Literally, it is a mountain path or mountain trail.
Other related words:

der Weg = way / path (more general)
Bergweg = mountain way/road (can be broader, maybe wider, sometimes even a small road)
Bergpfad suggests something narrower and more natural than Bergweg.

Why are the adjectives schmal and sicher not declined (no endings like -e, -er, etc.)?

In this sentence, schmal and sicher are predicate adjectives, used after the verb sein (to be):

Der Bergpfad ist schmal.
Der Bergpfad ist sicher.

Predicate adjectives in German do not take endings. They stay in the basic form.
Adjective endings appear when the adjective comes before a noun:

ein schmaler Bergpfad
ein sicherer Weg

Why is there no second ist in ist schmal, aber sicher? Could I say ist schmal, aber ist sicher?

German often leaves out repeated verbs when the meaning is clear. Here, the full form would be:

Der Bergpfad ist schmal, aber (er ist) sicher.

Dropping the second ist is normal and sounds more natural:
Der Bergpfad ist schmal, aber sicher.

You can say …aber er ist sicher, but …aber ist sicher (without er) sounds odd and is usually avoided.

Is the comma before aber necessary in ist schmal, aber sicher?

In this case, the comma before aber is optional, not strictly required.

We have one subject (Der Bergpfad) and one verb (ist) with two predicate adjectives, linked by aber.
For this type of contrast (schmal vs. sicher), you may write:
- Der Bergpfad ist schmal, aber sicher.
- Der Bergpfad ist schmal aber sicher.

Many writers prefer the comma here because it makes the contrast clearer, but both versions are accepted.

Why is there a comma before wenn?

The word wenn introduces a subordinate clause (wenn man langsam geht).
In German, subordinate clauses are always separated by a comma from the main clause.

So you must write:

Der Bergpfad ist schmal, aber sicher, wenn man langsam geht.
or with the clause at the front: Wenn man langsam geht, ist der Bergpfad schmal, aber sicher.

Why does the verb go to the end of the clause in wenn man langsam geht?

Wenn is a subordinating conjunction. In subordinate clauses introduced by words like wenn, weil, dass, obwohl, the conjugated verb goes to the end:

wenn man langsam geht
weil er müde ist
dass sie glücklich sind

In the main clause, the verb is in second position (Der Bergpfad ist schmal), but in the subordinate clause, it moves to the end (…wenn man langsam geht).

Why does the sentence use man instead of du or Sie?

The pronoun man is an impersonal pronoun meaning roughly one, you (in general), or people:

wenn man langsam geht ≈ if you go slowly / if one walks slowly (in general).

It sounds neutral and general, not directed at one specific person.
You could say wenn du langsam gehst to address one specific person informally, or wenn Sie langsam gehen in polite/formal address.

What is the difference between wenn man langsam geht and wenn du langsam gehst?

wenn man langsam geht = a general rule. It applies to anyone.
wenn du langsam gehst = speaking directly to one person in a familiar way (friends, family).

Meaning-wise, they can describe the same situation, but:

man → more neutral, general, often used in explanations or descriptions.
du → clearly aimed at a specific listener, more personal.

Is langsam here an adjective or an adverb? Why doesn’t it change form?

In wenn man langsam geht, langsam functions as an adverb (it describes how someone goes/walks).

In German, the adverb form of most adjectives looks exactly like the base adjective:

Adjective before a noun (with ending): ein langsamer Bergpfad
Adverb with a verb (no ending): man geht langsam

So langsam does not change here; that is normal for adverbial use.

Can I say wenn man geht langsam instead of wenn man langsam geht?

No, that would be incorrect word order in a subordinate clause.

In a wenn-clause, the conjugated verb must be at the end:

Correct: wenn man langsam geht
Incorrect: wenn man geht langsam

You can move langsam around within the verb bracket in some structures (e.g. with a separable verb), but the finite verb still stays at the end of the clause.

Can I move the wenn-clause to the beginning of the sentence?

Yes. Subordinate clauses like wenn man langsam geht can appear at the start:

Wenn man langsam geht, ist der Bergpfad schmal, aber sicher.

When you put the wenn-clause first, it counts as the first element, so in the following main clause the verb (ist) must come immediately after it (verb in second position).

Could I also say Der Bergpfad ist schmal, aber sicher, wenn du langsam gehst? Does that change the meaning?

Yes, that is grammatically fine:

Der Bergpfad ist schmal, aber sicher, wenn du langsam gehst.

Now the condition is clearly directed at you (one specific listener, informal).
With man, it sounds like a general rule; with du, it sounds more like personal advice: For you, it is safe if you go slowly.

Why is the sentence in the present tense geht and not something like ging?

The present tense in German is used:

for the present,
for general truths or rules, and
often even for near future events.

Here, wenn man langsam geht states a general condition or rule: whenever/if you go slowly, it is safe.
You could use ging in a narrative, story-like past context, but as a general statement, geht is the natural choice.