Learn Hardcore German: Die Ärztin erklärt, die Impfung habe nur leichte Nebenwirkungen. - The doctor female explains the vaccination has only mild side effects.

It is used because this is reported (indirect) speech. The sentence is reporting what the doctor says, not quoting her directly. In German, the Konjunktiv I is the standard way to mark that something is being reported rather than stated as the narrator’s own fact.

Pattern for haben:

Indicative present:
- er/sie/es hat (he/she/it has)
Subjunctive I present:
- er/sie/es habe

So die Impfung habe … signals: “The doctor says that the vaccine has …” without the writer fully committing to the truth of that statement.

What would the direct speech version of this sentence look like?

The reported sentence:

Die Ärztin erklärt, die Impfung habe nur leichte Nebenwirkungen.

corresponds to direct speech like:

Die Ärztin erklärt: Die Impfung hat nur leichte Nebenwirkungen.

In real written German, the second part would usually be put in quotation marks, but grammatically the key point is:

Direct speech (what she says literally): Die Impfung hat nur leichte Nebenwirkungen.
Reported speech (what someone reports about what she said): die Impfung habe nur leichte Nebenwirkungen.

So hat → habe is the change that marks the move from direct to indirect speech.

I learned that verbs go to the end in subordinate clauses. Why is habe in second position here?

You are right for typical subordinate clauses introduced by a conjunction like dass, weil, wenn, etc.:

…, weil die Impfung nur leichte Nebenwirkungen hat.
(Verb at the end.)

But in the sentence:

Die Ärztin erklärt, die Impfung habe nur leichte Nebenwirkungen.

the part die Impfung habe nur leichte Nebenwirkungen is a special kind of subordinate clause used in reported speech: an uneingeleiteter Nebensatz (a subordinate clause without a conjunction).

Such clauses usually keep main-clause word order:

Subject in first position (die Impfung)
Conjugated verb in second (habe)
Rest of the sentence afterwards

So functionally it is a subordinate clause (it depends on erklärt), but structurally it looks like a main clause with the verb in second position. That is normal in indirect speech in German.

Why is there no dass before die Impfung, and can we add it?

There is an implicit dass here. The clause

die Impfung habe nur leichte Nebenwirkungen

functions like:

dass die Impfung nur leichte Nebenwirkungen habe

German allows you to omit dass in many reported-speech sentences, especially in more formal or journalistic style.

You can absolutely say:

Die Ärztin erklärt, dass die Impfung nur leichte Nebenwirkungen habe.
(Still using Konjunktiv I → more formal / neutral, a bit distant.)

More commonly in everyday speech you will see:

Die Ärztin erklärt, dass die Impfung nur leichte Nebenwirkungen hat.
(Indicative → speaker presents it more as a plain fact.)

So:

Without dass: sounds more like classic reported speech, often in news: …, die Impfung habe …
With dass: more neutral; with hat it’s everyday style; with habe it’s careful, formal reported speech.

Could we use hat instead of habe here, and how would that change the style or meaning?

Grammatically you can say:

Die Ärztin erklärt, die Impfung hat nur leichte Nebenwirkungen.

This uses the indicative (hat) instead of the subjunctive (habe).

Differences:

habe (Konjunktiv I):
- Clear marker of indirect/reported speech
- Typical of news reports, formal writing, official statements
- Signals some distance: this is what she says, not necessarily what I, the reporter, claim as fact.
hat (indicative):
- Much more everyday and common in spoken language
- The boundary between reported and stated-as-fact becomes fuzzier
- In writing, style guides often prefer habe to keep that formal distance.

So hat is very natural in conversation, but habe is the “schoolbook” / journalistic standard for indirect speech.

How do the tenses work? Could we say erklärte or hat erklärt instead of erklärt?

Yes, you can change the tense of erklären in the main clause, and habe will usually stay the same in indirect speech if the statement is still valid:

Present:
- Die Ärztin erklärt, die Impfung habe nur leichte Nebenwirkungen.
  (She explains / is explaining that …)
Simple past:
- Die Ärztin erklärte gestern, die Impfung habe nur leichte Nebenwirkungen.
  (Yesterday she explained that …)
Present perfect:
- Die Ärztin hat erklärt, die Impfung habe nur leichte Nebenwirkungen.

In indirect speech, German generally does not shift the tense in the reported clause the way English does. You usually keep the same tense as in the original direct sentence and just change the mood (indicative → Konjunktiv I).

What do Die Ärztin and die Impfung tell us about gender and case?

Die Ärztin:

Ärztin is the feminine form of Arzt (doctor).
The suffix -in usually marks a female person: Lehrerin, Studentin, Nachbarin, etc.
Die Ärztin is nominative singular feminine, and it is the subject of the main clause.

die Impfung:

Impfung is also feminine; many nouns ending in -ung are feminine: die Zeitung, die Bewegung, die Meinung, etc.
In die Impfung habe …, die Impfung is the subject of the reported clause, so it is also nominative singular feminine.
The article die is the nominative singular feminine article (as well as the plural article, but here the noun is singular, so context tells you it is feminine singular).

So both Die Ärztin and die Impfung are feminine, nominative, singular subjects of their respective clauses.

Why is it leichte Nebenwirkungen and not leichten Nebenwirkungen?

The phrase nur leichte Nebenwirkungen is the direct object of habe (or of hat in direct speech):

The verb haben takes its object in the accusative case.
Nebenwirkungen is plural (side effects, more than one).
There is no article in front of leichte Nebenwirkungen (no die, einige, etc.).

For plural accusative without an article, the adjective ending is -e:

Nominative plural: leichte Nebenwirkungen
Accusative plural: leichte Nebenwirkungen

So leichte is correct here; leichten Nebenwirkungen would suggest dative plural (e.g. mit leichten Nebenwirkungen) or follow a different article pattern.

Mini comparison:

Nominative plural, no article: leichte Nebenwirkungen treten auf.
Accusative plural, no article: Die Impfung hat nur leichte Nebenwirkungen.
Dative plural, with article: mit den leichten Nebenwirkungen.

What exactly does nur mean here, and what happens if we move it?

In die Impfung habe nur leichte Nebenwirkungen, nur means only and it limits the type/degree of side effects:

only mild side effects, nothing more serious

Its scope is over leichte Nebenwirkungen.

Some other possible positions and their meanings:

Die Impfung hat nur leichte Nebenwirkungen.
→ Only the side effects are limited to being mild.
Nur die Impfung hat leichte Nebenwirkungen.
→ Only the vaccine (and not other treatments) has side effects.
Die Impfung hat leichte Nebenwirkungen nur am Anfang.
→ Only the time is limited (only at the beginning).

So nur generally comes right before the element it is restricting. In the original sentence, it restricts leichte Nebenwirkungen.

Is there any nuance difference between erklärt and a simpler verb like sagt here?

Yes, there is a nuance difference.

sagen (to say) is the most neutral, general verb for reporting speech:
- Die Ärztin sagt, die Impfung habe nur leichte Nebenwirkungen.
erklären (to explain) suggests:
- A bit more formality
- That the doctor is informing, possibly giving reasons or clarification
- Often appears in official statements, press releases, news reports

So:

sagt → she says (just reporting words)
erklärt → she explains / states / declares (more formal, slightly stronger, sometimes implying authority or expertise)

In many contexts, you can swap them, but erklärt here fits the idea of a doctor giving a professional, perhaps official, statement about the vaccine.