Learn Hardcore German: Wenn ich die laute Sirene höre, spüre ich plötzlich wieder meine alte Angst vor Krankenwagen. - When I hear the loud siren, I suddenly feel my old fear of ambulances again.

In the main clause (after the comma), word order changes back to verb-second:
… spüre ich plötzlich wieder meine alte Angst …
Here the verb (spüre) is in second position, as usual in main clauses.

Why is there a comma after höre?

German requires a comma between a subordinate clause and the main clause.

Wenn ich die laute Sirene höre, = subordinate clause
spüre ich plötzlich wieder meine alte Angst vor Krankenwagen. = main clause

Any construction with wenn, dass, weil, obwohl, etc., that is followed by a main clause must be separated by a comma.

Why is it wenn and not als or wann at the beginning?

All three can translate to when in English, but they are used differently:

wenn: for repeated, habitual, or general events (whenever) and also for future conditions.
- Wenn ich die Sirene höre, spüre ich Angst. = Whenever I hear the siren, I feel fear.
als: for a single event in the past.
- Als ich die Sirene hörte, hatte ich Angst. = When I heard the siren (that one time), I was afraid.
wann: for questions about time (direct or indirect).
- Wann hörst du die Sirene? = When do you hear the siren?

In the sentence you gave, the situation is general/repeated, so wenn is correct.

Why is it die laute Sirene and not something like der laute Sirene or die laut Sirene?

A few things are happening here:

Gender and case
- Sirene is feminine: die Sirene (nominative singular).
- In the sentence, Sirene is a direct object (I hear the siren), so it’s accusative feminine.
- For feminine nouns, nominative and accusative are both die. So die Sirene fits in both roles.
Adjective ending
- The adjective laut (loud) comes before a feminine accusative noun with a definite article (die), so it takes -e:
  - die laute Sirene
No adverb form here
- laut can be an adjective (eine laute Sirene) or an adverb (laut sprechen = speak loudly).
- Before a noun you need the adjective form with an ending: laute, not just laut.

So die laute Sirene is:
article (die) + adjective with correct ending (laute) + noun (Sirene).

Why is it meine alte Angst and not meinen alten Angst?

Again, this is about gender, case, and adjective endings:

Angst is feminine: die Angst.
Here, Angst is the direct object of spüre:
- Ich spüre meine alte Angst.

In German, feminine nouns have die / eine / meine in both nominative and accusative:

Nominative: meine alte Angst
Accusative: meine alte Angst

So you do not change to meinen (that’s masculine accusative: mein Hund → meinen Hund).

The adjective endings:

Possessive determiner mein- behaves like ein-.
Feminine accusative with mein-: meine alte Angst (adjective ending -e).

Why is spüre used instead of fühle for “I feel”? Are they different?

Both spüren and fühlen can mean to feel, but there are nuances:

spüren often emphasizes sensing something, noticing a physical or emotional reaction, sometimes suddenly or subtly.
- Ich spüre Angst. = I sense/feel fear.
- Often used for bodily sensations and internal reactions.
fühlen is more general, often about emotions and how you feel overall.
- Ich fühle Angst. / Ich fühle mich ängstlich. = I feel fear / I feel anxious.

In your sentence, spüre works well because it’s about suddenly noticing that old fear rising again:
… spüre ich plötzlich wieder meine alte Angst …
fühle would also be grammatically correct, just a bit more neutral.

Why are the adverbs in the order plötzlich wieder and not wieder plötzlich?

Both orders are possible, but the nuance changes slightly.

plötzlich wieder:
- plötzlich (suddenly) is closer to the verb idea: the experience comes suddenly.
- wieder (again) modifies spüre in the sense of “once more”.
- Reading: I (suddenly) (again) feel my old fear — the stress is on the suddenness of it coming back.
wieder plötzlich would sound more like “I again suddenly feel…” and is less idiomatic in this sentence.

In normal speech, plötzlich wieder is more natural because plötzlich (how it happens) tends to stand closer to the verb’s core meaning than wieder (frequency/repetition). German has flexible adverb order, but there are “more natural” patterns, and this is one of them.

Why is Krankenwagen not changed in the plural? Why not Krankenwägen or Krankenwagenen?

Krankenwagen belongs to a group of nouns where singular and plural look the same:

Singular: der Krankenwagen (the ambulance)
Plural: die Krankenwagen (the ambulances)

You see the plural only in the article:

der Krankenwagen = one ambulance (nominative singular)
die Krankenwagen = ambulances (nominative plural)

German has several plural patterns; this is one of them.
So in meine alte Angst vor Krankenwagen, Krankenwagen is plural, signaled by:

the context (fear of ambulances in general), and
the lack of an article (but it’s still a valid plural form).

Why does vor take the dative case here: vor Krankenwagen?

vor is one of the so‑called two-way prepositions (Wechselpräpositionen), which can take:

dative = location/state (where something is)
accusative = motion towards a place (where something goes to)

In Angst vor Krankenwagen, it’s not about moving or going in front of ambulances; it’s about having fear with respect to ambulances. This is a static relation, so dative is used.

Also, with psychological meanings (fear of X), vor almost always uses the dative:

Angst vor Hunden (fear of dogs)
Angst vor Prüfungen (fear of exams)
Angst vor Krankenwagen (fear of ambulances)

Why is it just vor Krankenwagen and not vor den Krankenwagen?

Both are possible, but they mean slightly different things:

vor Krankenwagen (no article)
- More general: fear of ambulances as a category, in general.
- Very natural in German for abstract/general fears.
vor den Krankenwagen (with article, dative plural)
- More specific: fear of particular, known ambulances (or the ones present in a given context).

In your sentence, the idea is a general old fear of ambulances, so the version without an article sounds more natural.

Why is German present tense used here instead of a past tense like hörte or spürte?

The German Präsens (present tense) is more flexible than English present:

It can describe:
- something happening right now
- habits/general truths
- whenever or every time situations (like here)
- even near future (with context)

Your sentence describes what typically happens whenever you hear a loud siren:

Wenn ich die laute Sirene höre, spüre ich …
= Whenever I hear the loud siren, I feel …

In English, we also often use the simple present for this kind of habitual statement, so the tenses match well here. If you were telling a specific past story, you might say:

Als ich die laute Sirene hörte, spürte ich plötzlich wieder meine alte Angst …
= When I heard the loud siren, I suddenly felt my old fear again.

Why are Sirene, Angst, and Krankenwagen capitalized, but laute, alte, plötzlich, wieder are not?

In German, all nouns are capitalized, regardless of where they appear in the sentence:

Sirene, Angst, Krankenwagen = nouns → capitalized.

Adjectives and adverbs are not capitalized (unless they begin the sentence or are part of a proper name):

laute, alte = adjectives → lowercase
plötzlich, wieder = adverbs → lowercase

So the capitalization simply marks which words are nouns.

Could the word order in the main clause be Ich spüre plötzlich wieder … instead of spüre ich plötzlich wieder …?

You can’t do that in this sentence, because of the wenn-clause at the beginning.

German main clauses have the finite verb in second position:

Wenn ich die laute Sirene höre, → the whole clause is position 1.
Then comes the verb of the main clause in position 2: spüre.
Then the subject: ich.
Then other elements: plötzlich wieder meine alte Angst vor Krankenwagen.

So:

Wenn ich die laute Sirene höre, spüre ich plötzlich wieder meine alte Angst vor Krankenwagen. ✅

If you want ich first, you must drop or move the wenn-clause:

Ich spüre plötzlich wieder meine alte Angst vor Krankenwagen, wenn ich die laute Sirene höre. ✅

But with the wenn-clause at the start, keeping spüre in second position is mandatory; Ich spüre … immediately after that would break the verb-second rule.