Learn Hardcore German: Der Zeuge bleibt ruhig und erklärt der Polizei genau, wie der Fahrer gebremst hat. - The witness stays calm and explains to the police exactly how the driver braked.

bleibt ruhig = stays / remains calm. It emphasizes that the witness continues to be calm, even though something stressful is happening.
ist ruhig = is calm. This just describes the state, without the idea of “continuing to stay that way”.

German commonly uses bleiben + adjective (bleibt ruhig, bleibt freundlich, bleibt still) to express “remain X” in situations where English might just say “is X” or “stays X”.

So in this sentence, bleibt ruhig underlines that the witness keeps his composure instead of panicking.

What grammatical role does „Der Zeuge“ have here?

Der Zeuge is the subject of the sentence.

It answers the question “Who stays calm and explains…?” → Der Zeuge.
It is in the nominative case.
The verbs bleibt and erklärt are both actions performed by this subject.

So structurally:

Subect (Nominative): Der Zeuge
Verbs: bleibt, erklärt

Why is it „erklärt der Polizei“ and not „erklärt die Polizei“?

Because „der Polizei“ is in the dative case, not nominative or accusative.

The verb erklären typically follows this pattern:

jemandem etwas erklären
= to explain something to someone

So:

jemandem → dative (the person who receives the explanation)
etwas → accusative (the thing that is explained)

In this sentence:

der Polizei = to the police → dative (indirect object)
the thing explained is the „wie…“ clause (see below).

If you said „erklärt die Polizei“ (accusative), it would sound like “explains the police”, i.e. explains the police themselves, which is wrong here.

Why is it „der Polizei“ and not „die Polizei“? Isn’t Polizei feminine?

Yes, Polizei is feminine, but the article changes with the case.

For a feminine noun like die Polizei:

Nominative: die Polizei
Accusative: die Polizei
Dative: der Polizei
Genitive: der Polizei

In the sentence we have „erklärt der Polizei“, and der Polizei is dative (indirect object → “to the police”), so the correct form is der, not die.

What exactly is „genau“ doing here, and why is it placed after „der Polizei“?

Genau here means “exactly / in detail / precisely”. It describes how the witness explains something.

Word order:

erklärt der Polizei genau
literally: explains to the police exactly

This is a neutral and very common position:

verb (erklärt)
indirect object (der Polizei)
adverb (genau)

You could also say:

erklärt genau der Polizei, … – This is grammatical, but now genau sounds like it is emphasizing the police (exactly the police, and not someone else); that is usually not what you want here.
erklärt genau, wie… – still understandable, but it sounds like “explains in detail how…” without explicitly saying to whom before the „wie“-clause. Less clear rhythmically.

So „erklärt der Polizei genau, …“ is the most natural order for “explains to the police exactly how…”.

Why is there a comma before „wie der Fahrer gebremst hat“?

Because „wie der Fahrer gebremst hat“ is a subordinate clause (Nebensatz).

In German, subordinate clauses introduced by words like:

dass (that),
weil (because),
wenn (if/when),
ob (whether),
wie (how),

are normally separated from the main clause with a comma.

So:

Main clause: Der Zeuge bleibt ruhig und erklärt der Polizei genau, …
Subordinate clause: wie der Fahrer gebremst hat.

The comma marks the beginning of that subordinate clause.

Why is the word order „wie der Fahrer gebremst hat“ and not „wie der Fahrer hat gebremst“?

In subordinate clauses (introduced here by wie), German puts the finite verb at the end of the clause.

Main clause word order: second position for the finite verb.
- Der Fahrer hat gebremst. (hat in 2nd position)
Subordinate clause word order: verb at the end.
- …, wie der Fahrer gebremst hat.

Because this is the Perfekt tense, we have two verb forms:

hat (finite auxiliary)
gebremst (past participle)

In subordinate clauses, both go to the end, with the participle directly before the finite verb:

… wie der Fahrer gebremst hat.

So „wie der Fahrer hat gebremst“ is ungrammatical.

Why is it „gebremst hat“ (perfect tense) and not „bremste“ (simple past)?

German uses the Perfekt (hat gebremst) much more frequently in spoken language than the Präteritum (bremste), especially for regular verbs like bremsen.

Der Fahrer bremste. (simple past)
– Correct, but sounds more written/formal or narrative.
Der Fahrer hat gebremst. (present perfect)
– Typical in everyday speech and also very common in written reports that imitate spoken style.

In this sentence, the report sounds like something you might say when telling the police what happened, so „hat gebremst“ is the natural choice.

What grammatical function does the clause „wie der Fahrer gebremst hat“ have?

The whole clause „wie der Fahrer gebremst hat“ functions as the direct object of „erklärt“.

The pattern is:

Der Zeuge erklärt der Polizei [was genau?]
→ wie der Fahrer gebremst hat.

So:

der Polizei = indirect object (dative) → to whom he explains
wie der Fahrer gebremst hat = direct object (content) → what he explains

Why is „der Fahrer“ here in the form der, not den?

Because der Fahrer is the subject of the subordinate clause „wie der Fahrer gebremst hat“, so it is in the nominative case.

Inside that clause:

Wer hat gebremst? → der Fahrer
Verb: hat (auxiliary) + gebremst

If it were „den Fahrer“, that would be accusative, used for a direct object. But here, the driver is not receiving an action; he is the one doing the braking, so nominative der is correct.

Why is „wie“ used here and not „dass“?

Wie and dass introduce different kinds of content clauses.

wie = how, it introduces a clause about manner or way something is done.
- er erklärt, wie der Fahrer gebremst hat
  = he explains how the driver braked (e.g. slowly, suddenly, several times, etc.)
dass = that, it introduces a clause that states a fact.
- er erklärt, dass der Fahrer gebremst hat
  = he explains that the driver braked (simply the fact that there was braking).

In your sentence, the focus is on the manner and details of the braking, not just the fact that it happened, so „wie“ is the right choice.

Is „ruhig“ here an adjective or an adverb? In English it would be “calm” vs “calmly”.

In German, the same word form is used where English distinguishes between adjective and adverb.

Predicate adjective after sein/werden/bleiben:
- Er bleibt ruhig. = He remains calm.
- Sie ist müde. = She is tired.

German does not add an ending like -ly:

no „ruhig-ly“ form, just ruhig.

So grammatically you can call ruhig here a predicative adjective after bleiben, but functionally it also answers “How does he remain?” (in a way like an adverb would in English). German simply doesn’t make a form difference between the two in this position.