Learn Hardcore German: Das Huhn flattert laut, wenn der Bauer den Stall betritt. - The chicken flutters loudly when the farmer enters the stable.

There is no logical reason for this from an English point of view; it’s simply part of the word’s dictionary entry. In German, every noun has a grammatical gender (masculine, feminine, or neuter) that you have to learn together with the noun.

Why does the verb flattert come in the second position in Das Huhn flattert laut?

In main clauses, German follows the rule “the conjugated verb is in position 2” (Verb-second, or V2 word order).

In Das Huhn flattert laut:

Das Huhn = first element (subject)
flattert = conjugated verb in second position
laut = everything else (adverbial)

You can move other elements to the first position, but the conjugated verb will still stay second, e.g.:

Laut flattert das Huhn.
Jeden Morgen flattert das Huhn laut.

In all these, the finite verb flattert is in second position.

Why is there a comma before wenn?

Wenn introduces a subordinate clause (wenn der Bauer den Stall betritt). In German, subordinate clauses are always set off by a comma from the main clause.

So you have:

Main clause: Das Huhn flattert laut
Subordinate clause: wenn der Bauer den Stall betritt

Rule: Every subordinate clause is separated from the main clause by a comma.

Why does the verb go to the end in wenn der Bauer den Stall betritt?

In subordinate clauses introduced by words like wenn, dass, weil, obwohl etc., the conjugated verb goes to the end of the clause (Verb-final word order).

Structure of this wenn-clause:

wenn = subordinating conjunction
der Bauer = subject (nominative)
den Stall = object (accusative)
betritt = conjugated verb at the end

So: wenn der Bauer den Stall betritt, not wenn der Bauer betritt den Stall.

This verb-final order is a central feature of German subordinate clauses.

Could we also say Wenn der Bauer den Stall betritt, flattert das Huhn laut? Does that change the meaning?

Yes, that word order is perfectly correct:

Wenn der Bauer den Stall betritt, flattert das Huhn laut.

Here, the wenn-clause comes first. Because a whole clause is now in position 1, the main-clause verb flattert must still stand in position 2 of the main clause, so you get:

Wenn der Bauer den Stall betritt (subordinate clause = entire “position 1”)
flattert (main-clause verb in 2nd position)
das Huhn laut (rest)

The meaning is essentially the same; putting the wenn-clause first just emphasizes the condition/time a bit more.

What is the difference between wenn and als? Could I say Als der Bauer den Stall betritt?

Wenn and als are not interchangeable:

wenn (with present tense here) expresses repeated or general situations:
- Das Huhn flattert laut, wenn der Bauer den Stall betritt.
  = Whenever / every time the farmer enters the coop, the hen flaps loudly.
als is used (with past tense) for one specific event in the past:
- Das Huhn flatterte laut, als der Bauer den Stall betrat.
  = The hen flapped loudly when the farmer entered the coop (on that one occasion).

So in your present-tense, habitual sentence, wenn is correct. With als you would normally also switch to a past tense.

Why is it der Bauer but den Stall? What’s going on with the articles?

Both Bauer and Stall are masculine nouns:

der Bauer (farmer)
der Stall (coop, stable)

But German marks case on the article. In the wenn-clause:

der Bauer is the subject → nominative → masculine nominative article der
den Stall is the direct object of betritt → accusative → masculine accusative article den

So the forms of the definite article for a masculine noun are:

Nominative: der Bauer (subject)
Accusative: den Stall (direct object)

What is the infinitive of betritt, and why doesn’t the prefix separate like in tritt ein?

The infinitive is betreten.

betreten = to enter (transitive verb; takes a direct object)
betritt = 3rd person singular present: er/sie/es betritt den Stall

The prefix be- here is an inseparable prefix. Inseparable prefixes (be-, ver-, zer-, er-, ent-, emp-, ge-, miss-) never split off and move to the end of the clause.

So:

er betritt den Stall (inseparable) but:
er tritt in den Stall ein (separable prefix ein- → eintritt, tritt … ein)

Both sentences can describe the same action, but betreten sounds a bit more formal/bookish; eintreten is more everyday.

Why is laut placed after flattert? Could I say Das Huhn laut flattert?

The normal, neutral word order is:

Das Huhn flattert laut.

German tends to place manner adverbs (how something happens) like laut after the verb and its objects.

Das Huhn laut flattert is grammatically possible in poetry or very marked emphasis, but in everyday speech it sounds odd or at least very strongly emphasized on laut.

More natural variations are:

Das Huhn flattert sehr laut.
Laut flattert das Huhn. (fronted for emphasis on “loudly”)

Is flattert just the present tense? Does it mean it’s happening right now or habitually?

Flattert is 3rd person singular present tense of flattern.

German present tense can express:

Something happening right now
- Das Huhn flattert laut. = The hen is flapping loudly (right now).
A repeated/habitual action
- Das Huhn flattert laut, wenn der Bauer den Stall betritt.
  = The hen flaps loudly whenever the farmer enters the coop.

In this sentence, the combination with wenn makes it clearly habitual / repeated.

Why is it das Huhn flattert and not das Huhn flattert sich or something similar? Doesn’t German use reflexive pronouns more?

Some German verbs are reflexive and need a sich (e.g. sich freuen, sich beeilen), but flattern is not reflexive in this meaning. It simply means “to flap (wings), to flutter,” and takes no object or reflexive pronoun here.

So the pattern is:

Subject + finite verb:
- Das Huhn flattert.
- Der Vogel flattert.

Adding sich (Das Huhn flattert sich) is ungrammatical in standard German in this sense.

Is Stall specifically a chicken coop, or more general?

Stall is a general word for an animal enclosure/building: stable, barn, pen, sty, coop — the exact English word depends on the animal and context.

For chickens specifically, you can also say:

Hühnerstall = chicken coop

In your sentence, because we have das Huhn, it’s clear from context that der Stall is the hen’s coop.