Learn Hardcore German: Der Arzt verlangt heute eine Überweisung, sonst behandelt er den Patienten nicht. - The doctor requires a referral today; otherwise he will not treat the patient.

Because Überweisung is feminine (die Überweisung). The accusative singular feminine article is eine (same as nominative). It’s the direct object of verlangt in the first clause.

Why den Patienten (with -en) and not den Patient or dem Patienten?

Patient is a weak (n‑declension) masculine noun: it adds -en in every case except nominative singular.
It’s the direct object of behandelt, so accusative singular: den Patienten.
Forms: nominative der Patient, accusative den Patienten, dative dem Patienten, genitive des Patienten.

What exactly does sonst do here?

Sonst means “otherwise/or else” and links two main clauses. It’s a conjunctional adverb (not a subordinating conjunction), so the following clause still has verb‑second word order: sonst behandelt er .... Synonyms: ansonsten, andernfalls.

Why is it sonst behandelt er and not sonst er behandelt?

German main clauses are verb‑second. When sonst occupies the first slot, the finite verb (behandelt) must come next, followed by the subject (er).

Where does nicht go, and what’s the difference between den Patienten nicht and nicht den Patienten?

Neutral negation of the action: place nicht late, after objects/complements → Er behandelt den Patienten nicht.
Contrastive negation of the object (he treats someone else, but not this patient): Er behandelt nicht den Patienten, sondern …

Could I use kein instead of nicht here?

No. You’re negating the action, so use nicht. Kein negates a noun phrase: Er behandelt keinen Patienten (= he treats no patients at all).

Why choose verlangt? How is it different from fordert, braucht, or bittet (um)?

verlangen: require/demand (neutral; typical for a rule/policy).
fordern: demand strongly/insistently; more forceful, often political/legal.
brauchen: need; states necessity, not that someone is being asked to provide it.
(jemanden) um etwas bitten: politely request. Here, verlangt matches a practice’s requirement toward the patient.

Does Überweisung also mean a bank transfer?

Yes. die Überweisung can mean:

medical referral slip
bank transfer Context decides: with Arzt, behandeln → medical; with Geld, Konto, Bank → financial.

Why is there a comma before sonst?

Because two independent main clauses are being separated. Sonst is an adverb inside the second main clause, so a comma (or a semicolon) is required between the clauses.

Can I start the sentence with Heute?

Yes: Heute verlangt der Arzt eine Überweisung, sonst behandelt er den Patienten nicht. The finite verb stays in second position after Heute.

Could I rewrite it with wenn instead of sonst?

Yes. For example: Wenn der Patient heute keine Überweisung hat, behandelt der Arzt ihn nicht. With wenn, the verb in that clause goes to the end (hat).

Why der Arzt and not der Doktor?

der Arzt / die Ärztin is the neutral job title and standard in writing.
der Doktor is either a title holder or a colloquial way to refer to the physician. In neutral prose, prefer Arzt.

Can I say vom Patienten with verlangen?

Yes: Der Arzt verlangt vom Patienten eine Überweisung. Pattern: etwas von jemandem verlangen (accusative for the thing, dative after von for the person).

Why present tense here? Could I use the future?

German often uses the present for rules and near‑future. You can use future for a predictive tone: ..., sonst wird er den Patienten nicht behandeln. The meaning is very close.

Is the position of heute okay? Could it go elsewhere?

Yes. Natural variants:

Der Arzt verlangt heute eine Überweisung. (neutral)
Heute verlangt der Arzt eine Überweisung. (emphasis on “today”)
Der Arzt verlangt eine Überweisung heute. (possible but marked; used for special emphasis)

Could I replace den Patienten with ihn?

Yes, if the referent is clear: ..., sonst behandelt er ihn nicht. Using the noun again is fine for clarity in an isolated sentence; in context, a pronoun is more natural.