Cuando me duele la nariz, respiro vapor con calma.

Spanish uses the verb doler like gustar: the thing that hurts is the grammatical subject, and the person who feels the pain is an indirect object. So:

• Me duele la nariz. = The nose hurts me. Here, la nariz is the subject (singular), and me is the person affected.

"Mi nariz duele" is grammatically possible but sounds unusual and emphatic, like "My nose hurts (as opposed to something else)." The neutral, natural way is Me duele la nariz. You can also front the subject: La nariz me duele, still less common than the standard order.

For emphasis on the person, add a prepositional phrase: A mí me duele la nariz.

With body parts, Spanish generally uses the definite article (el/la/los/las) instead of a possessive when the possessor is clear from a pronoun:

• Me duele la nariz. (My nose hurts.)
• Le duele la garganta. (His/her throat hurts.)

You only use a possessive for contrast or clarity when needed: Me duele mi nariz, no la tuya. Without that contrast, mi nariz sounds odd.

Agreement is with the thing that hurts (the subject), not the person:

• Singular subject: Me duele la nariz / la espalda / el hombro.
• Plural subject: Me duelen los ojos / los pies / las manos.
• Multiple items: Me duelen la cabeza y la garganta.

No. Me here is an indirect object pronoun, not a reflexive. The verb doler works like gustar:

• Subject: the body part (la nariz).
• Indirect object: the experiencer (me/te/le…).

Compare:

• Me duele la nariz. = My nose hurts (to me).
• Me lastimé la nariz. = I hurt/injured my nose. (This one is reflexive and means you caused the injury.)

It depends on meaning:

• Habitual/general time (≈ “whenever”): use indicative.
  • Cuando me duele la nariz, respiro vapor con calma.
• Future/uncertain time (a condition that hasn’t happened yet): use subjunctive in the cuando-clause and future (or ir a + infinitive) in the main clause.
  • Cuando me duela la nariz, respiraré vapor con calma.
  • Cuando me duela la nariz, voy a respirar vapor con calma.

Yes, it’s standard when the dependent clause comes first:

• Cuando me duele la nariz, respiro vapor con calma. If the main clause comes first, you normally omit the comma:
• Respiro vapor con calma cuando me duele la nariz.

All are understandable, but preferences vary:

• Respiro vapor = I breathe steam; common and clear in everyday speech.
• Inhalo vapor = I inhale steam; slightly more technical/formal.
• Very natural phrasing for the remedy: Hago inhalaciones de vapor or hago vaporizaciones. Depending on country, you also hear hacer vahos (“steam inhalations”).

Avoid saying just hago vapor; say hago inhalaciones de vapor or tomo un baño de vapor (steam bath/sauna) depending on context.

Mass/uncountable nouns often appear without an article when speaking in general:

• Respiro vapor = I breathe steam (in general). Use a definite article for specific steam:
• Respiro el vapor de la ducha/de la olla. = I breathe the steam from the shower/pot.

Typical and most natural positions:

• After the verb or at the end: Respiro vapor con calma.
• At the start for emphasis: Con calma, respiro vapor. Avoid splitting noun and its complement: Respiro con calma vapor is less natural.

It’s unusual. Natural options:

• Me duele la nariz. (best)
• Tengo dolor en la nariz. (possible but less common) For specific sensations:
• Me arde la nariz. (it burns)
• Tengo la nariz congestionada/irritada. (stuffy/irritated)

Note: Tengo dolor de cabeza is idiomatic, but with “nariz” it’s not commonly used.

Yes. Besides inhalar/respirar vapor, you may hear:

• Hacer inhalaciones de vapor (widely used)
• Hacer vaporizaciones
• Hacer vahos (common in several countries; “vaho” = steam/foggy breath) All are understood in Latin America.