Learn Hardcore German: Der Arzt sagt, mein Hals sei gesund, aber mein Kopf wirke müde. - The doctor says my throat is healthy, but my head seems tired.

Sei is the subjunctive I (Konjunktiv I) form of sein. It’s used here because this is reported/indirect speech (what the doctor says) rather than a direct quote.

Direct speech: Der Arzt sagt: „Dein Hals ist gesund, aber dein Kopf wirkt müde.“
Indirect speech: Der Arzt sagt, mein Hals sei gesund, aber mein Kopf wirke müde.

Using sei shows that the speaker is reporting the doctor’s statement, not stating it as their own fact. This is very common in written German (news, reports, formal writing) to show distance from the original statement.

What exactly is sei grammatically? How is it formed?

Sei is the present tense, subjunctive I (Konjunktiv I), 3rd person singular form of sein.

Present Konjunktiv I of sein:

ich sei
du seiest
er/sie/es sei
wir seien
ihr seiet
sie/Sie seien

It contrasts with:

Indicative present: ich bin, du bist, er ist, …
Subjunctive II (Konjunktiv II): ich wäre, du wärst, er wäre, …

In indirect speech in standard German, you normally use Konjunktiv I if its forms are clearly distinct from the indicative. Here sei is clearly different from ist, so sei is preferred.

Why is it wirke and not wirkt in mein Kopf wirke müde?

The same reason as with sei: wirke is the Konjunktiv I form of wirken in the 3rd person singular, used for indirect speech.

Indicative present, 3rd person singular: er wirkt
Konjunktiv I present, 3rd person singular: er wirke

Because the whole thing is what the doctor says, both verbs (sei, wirke) are put in Konjunktiv I to mark reported speech consistently:

Der Arzt sagt, mein Hals sei gesund, aber mein Kopf wirke müde.

What does wirken mean here? How is it different from sein or aussehen?

In this context, wirken means “to seem / to come across / to give the impression of”.

Er ist müde. – He is tired (a factual statement about his state).
Er wirkt müde. – He seems tired / He comes across as tired (how he appears to others).
Er sieht müde aus. – He looks tired (visually).

So mein Kopf wirke müde means something like:

“my head seems tired” / “my head appears tired”.

The doctor is judging the impression your head gives (maybe your facial expression), not stating an objective measurable property like “is broken” or “is swollen”.

Can I just say ist and wirkt instead of sei and wirke?

Yes, especially in everyday spoken German you will often hear the indicative instead of Konjunktiv I:

Der Arzt sagt, mein Hals ist gesund, aber mein Kopf wirkt müde.

This is grammatically fine and very common in conversation.

However:

In more formal or written German (newspapers, official reports), Konjunktiv I (sei, wirke) is preferred to make it clear that it is someone else’s statement.
Using sei / wirke also slightly distances the speaker from the truth of the statement: you are reporting what was said, not necessarily confirming it.

So:

Everyday, neutral: … sagt, mein Hals ist gesund …
Formal / reporting style: … sagt, mein Hals sei gesund …

Could I add dass and say Der Arzt sagt, dass mein Hals gesund ist … instead?

Yes, that is perfectly correct:

Der Arzt sagt, dass mein Hals gesund ist, aber mein Kopf müde wirkt.

Points to note:

With dass, German most often uses the indicative (ist, wirkt) in everyday language.
You can also use Konjunktiv I after dass:
- …, dass mein Hals gesund sei, aber mein Kopf müde wirke. This sounds quite formal/literary.
In the original sentence, dass is simply omitted, which German allows after verbs like sagen, glauben, denken, meinen:
- Er sagt, (dass) mein Hals sei gesund.

So you have several stylistic options, all grammatical with slightly different levels of formality.

Is mein Hals sei gesund a subordinate clause? Why is the word order like a main clause?

Logically, mein Hals sei gesund is the content of what the doctor says, so functionally it behaves like a subordinate clause (a “that”-clause).

However, German often uses main-clause word order (verb in 2nd position) for indirect speech without dass:

Der Arzt sagt, mein Hals sei gesund.
- mein Hals = first position
- sei = verb in 2nd position
- gesund = rest of the clause

Grammars call this an “uneingeleiteter Nebensatz mit Verbzweitstellung” (subordinate clause without a conjunction, with V2 word order) or sometimes just an “indirect-speech main clause”.

So:

Function: like “that my throat is healthy”.
Form: looks like a main clause (V2) because of the omitted dass and the use of Konjunktiv I.

Why is there a comma before aber?

The comma is required because aber here connects two full clauses:

mein Hals sei gesund
mein Kopf wirke müde

In German, when aber joins two independent clauses, you must put a comma:

…, mein Hals sei gesund, aber mein Kopf wirke müde.

Compare:

Er ist gesund, aber müde.
Still a comma, because there are two clauses:
- er ist gesund
- (er) ist müde (subject & verb implied, but structure is still clausal)

Why is mein repeated before Kopf? Could you say …, aber Kopf wirke müde?

You normally cannot drop the determiner (article/possessive) in a new clause like that in standard German.

Correct:
Der Arzt sagt, mein Hals sei gesund, aber mein Kopf wirke müde.
Wrong / very unidiomatic:
✗ …, aber Kopf wirke müde.

Reasons:

In German, a bare singular noun like Kopf usually needs a determiner (der, mein, sein, etc.).
You’re starting a new clause after aber, so you repeat the possessive to make it clear it’s your head.

You can share the possessive within a single noun phrase:

Mein Hals und Kopf tun weh.

But once you start a new finite clause (…, aber … wirke müde), you normally reintroduce mein:

…, aber mein Kopf wirke müde.

Why do gesund and müde have no adjective endings?

Because here they are predicative adjectives, used after verbs like sein, wirken, werden, aussehen, scheinen, etc.

Predicative adjectives in German do not take endings:

Mein Hals ist gesund.
Mein Kopf wirkt müde.
Die Kinder sind laut.
Er scheint nett.

Adjective endings (-e, -en, -er, -es, …) appear when the adjective directly modifies a noun (attributive position):

ein gesunder Hals
mein müder Kopf
laute Kinder
ein netter Mann

So:
Prädikativ: Hals ist gesund, Kopf wirke müde → no endings.

What happens if I change sagt to past tense sagte? Do sei and wirke change?

You can absolutely say:

Der Arzt sagte, mein Hals sei gesund, aber mein Kopf wirke müde.

In German indirect speech, the tense inside the reported clause (sei / wirke) does not automatically shift when the reporting verb goes to the past. We still use Konjunktiv I to represent whatever tense the original speaker used.

So:

Direct: „Dein Hals ist gesund, aber dein Kopf wirkt müde.“
Indirect:
- Present: Der Arzt sagt, dein Hals sei gesund, aber dein Kopf wirke müde.
- Past: Der Arzt sagte, dein Hals sei gesund, aber dein Kopf wirke müde.

If you needed to express past-before-past, you could use a perfect form in Konjunktiv I (e.g. gesund gewesen sei), but that’s another layer of complexity.

Why are Hals, Kopf, and Arzt capitalized?

Because in German, all nouns are capitalized.

In this sentence:

der Arzt – Arzt = noun, capitalized
mein Hals – Hals = noun, capitalized
mein Kopf – Kopf = noun, capitalized

This is a general rule:

die Gesundheit, der Patient, die Krankheit, das Problem, etc.

Adjectives, verbs, and most other words are not capitalized unless they start a sentence or are part of a proper name.