Learn Hardcore German: Wenn wir später an den Bach zurückkehren, wirkt er wieder ruhig, als hätte es nie eine Schwierigkeit gegeben. - When we return to the creek later, it seems calm again, as if there had never been any difficulty.

Zu dem Bach / zum Bach would also be possible and correct, but it’s a bit less specific: more like “(back) to the area of the stream” rather than “(back) to the stream’s edge/bank.”
An den Bach paints the image of going right up to the stream.

Why is the verb wirkt used instead of ist in wirkt er wieder ruhig?

Wirken in this context means “to seem / to appear”, not “to work.”

Er ist ruhig. – It is calm. (neutral statement of fact)
Er wirkt ruhig. – It appears calm / It seems calm. (emphasis on how it looks to us)

So wirkt er wieder ruhig suggests we are describing our impression of the stream, not just stating an objective fact. It adds a slight nuance of subjectivity.

Why is the present tense used (kehren … zurück, wirkt) if the sentence talks about later?

German very often uses the present tense for future events when there is a time expression that makes the future meaning clear, such as später, morgen, nächste Woche.

Wenn wir später an den Bach zurückkehren, …
literally: If/When we later return to the stream…
but functionally: When we return to the stream later…

Using werden is possible but not necessary:

Wenn wir später an den Bach zurückkehren werden, … – grammatically possible but sounds heavier and is less common in everyday speech.

So the present tense here is normal and idiomatic for a future event.

What does wenn do in this sentence, and how does it affect the word order?

Wenn introduces a subordinate clause (a dependent clause) and means when or if here.

In the wenn‑clause, the conjugated verb goes to the end:
- Wenn wir später an den Bach zurückkehren, …
Because the subordinate clause comes first, the main clause must start with the verb (inversion):
- …, wirkt er wieder ruhig, …
  - verb (wirkt) – subject (er) – rest.

So the structure is:

[Wenn … zurückkehren], [wirkt er wieder ruhig].

This is a standard pattern: subordinate clause first → verb comes first in the main clause.

Why does the sentence later use als (als hätte es …) instead of wenn again? What’s the difference?

The two words have different functions here:

Wenn = when / if
– introduces a time or condition.
Als in als hätte es nie … gegeben is part of a comparative construction meaning “as if”. It’s essentially a shortened form of als ob:
- …, als (ob) es nie eine Schwierigkeit gegeben hätte.
  – …, as if there had never been any difficulty.

So:

First part: Wenn → “When/If we later return …”
Second part: als → “as if there had never been any difficulty.”

They are not interchangeable here.

In als hätte es nie eine Schwierigkeit gegeben, why is it hätte (subjunctive) and not hatte (simple past) or hat (present)?

Hätte is Konjunktiv II (subjunctive II), used here to show an unreal / imaginary comparison: as if X had happened, although it actually did not.

als hätte es nie eine Schwierigkeit gegeben
→ as if there had never been any difficulty (but in reality there was).

If you said:

als hätte es nie eine Schwierigkeit – ungrammatical: you’re missing the participle gegeben and thus the perfect form.
als hatte es nie eine Schwierigkeit gegeben – sounds wrong; hatte is simple past, not the subjunctive needed for this unreal comparison.
als ob es nie eine Schwierigkeit gegeben hätte – fully spelled‑out version, also using hätte (subjunctive II).

So hätte … gegeben expresses a hypothetical/irrealis past situation.

Why is it als hätte es nie eine Schwierigkeit gegeben and not als ob es nie eine Schwierigkeit gegeben hätte?

Both are grammatically correct and mean the same:

als hätte es nie eine Schwierigkeit gegeben
- als
  - Konjunktiv II with inversion (verb first: hätte es …)
- This is a shortened form of als ob es … hätte and is quite common in written German.
als ob es nie eine Schwierigkeit gegeben hätte
- explicit als ob
- standard subordinate clause word order: subject–objects–hätte at the end.

The difference is mainly style:

als hätte … can sound a bit more literary or compact.
als ob … hätte is slightly more explicit and “textbook‑like.”

Meaning-wise, there is no real difference here.

What does es stand for in es nie eine Schwierigkeit gegeben?

Here es is a dummy subject, like “there” in English:

Es hat eine Schwierigkeit gegeben.
→ There has been a difficulty.

German uses es with geben in this sense:

Es gibt eine Schwierigkeit. – There is a difficulty.
Es gab eine Schwierigkeit. – There was a difficulty.
Es hätte nie eine Schwierigkeit gegeben. – There would never have been a difficulty.

So es doesn’t refer to a real thing; it’s just the grammatical subject required by geben in this “there is/there was” structure.

Why is Schwierigkeit singular with eine, and not plural Schwierigkeiten?

German often uses the singular eine Schwierigkeit idiomatically to mean:

“a difficulty / any difficulty / any problem at all”

So:

als hätte es nie eine Schwierigkeit gegeben
→ as if there had never been (even) a single difficulty / any difficulty at all.

You could also say:

als hätte es nie Schwierigkeiten gegeben
→ as if there had never been any difficulties.

Both are correct; the singular with eine tends to highlight the idea of “not even one”.

Why is the word order es nie eine Schwierigkeit gegeben and not es gegeben nie eine Schwierigkeit?

The verb phrase here is part of hätte … gegeben (a perfect form with an auxiliary + participle):

The auxiliary (hätte) appears earlier (after als) because of the special als hätte structure.
The participle gegeben must go to the end of the clause.
Adverbs like nie and objects like eine Schwierigkeit go before the final participle.

So the logical skeleton is:

hätte – es – nie eine Schwierigkeit – gegeben

Putting gegeben anywhere but last would break the standard verb-final rule for participles in subordinate clauses:

✗ es gegeben nie eine Schwierigkeit – incorrect word order.

Is there a difference between wieder ruhig and ruhig wieder? Could we swap them?

Yes, there is a clear preference:

wirkt er wieder ruhig – natural and idiomatic
wirkt er ruhig wieder – sounds odd or wrong in this meaning.

General tendency in German: adverbs like wieder (again) usually come before adjectives they modify in predicative use:

Er ist wieder ruhig.
Es wird wieder warm.

Ruhig wieder would need a different context to sound natural and would not be used here.

What is the difference between später and nachher? Could we say Wenn wir nachher an den Bach zurückkehren?

You could say:

Wenn wir nachher an den Bach zurückkehren, …

and it would be grammatically fine.

Nuance:

später – later in a fairly general sense; can be more neutral and is common in narrative style.
nachher – afterwards / a bit later (from now); often used in spoken language for relatively near future (“after this, after we finish this thing”).

In this more narrative, descriptive sentence about returning to a stream, später often feels more natural and slightly more literary/neutral than nachher.

Why is it zurückkehren and not just kehren? And is the prefix ever separated?

Zurückkehren is the normal verb for “to return / to go back”.

zurück = back
kehren = to turn

Together: zurückkehren = to turn back, return.

Using only kehren (kehren wir an den Bach) is either:

unusual in this meaning, or
interpreted as another verb kehren (to sweep), which would be confusing.

About separation:

Zurückkehren is a separable prefix verb.
In the sentence you gave, it appears in the infinitive inside a subordinate clause, so it stays together:
- wenn wir … zurückkehren
In a simple main clause, the prefix would separate:
- Wir kehren an den Bach zurück.

So yes, normally the prefix separates in main clauses, but not in infinitive forms and not in certain subordinate clause structures.