Cuando subo las escaleras rápido, me duele la espalda.

Spanish uses indicative vs. subjunctive after cuando depending on the meaning:

• Indicative (as in cuando subo) is used for:

  • Habitual actions
    
    • Cuando subo las escaleras rápido, me duele la espalda.
      = Any time I go up the stairs quickly, this happens.
  • Past actions that are seen as real and completed.

• Subjunctive is used when the action is:

  • In the future and not yet realized, or
  • Hypothetical/uncertain
    
    • Cuando suba las escaleras, me dolerá la espalda.
      = When I go up the stairs (later / in the future), my back will hurt.

Here the sentence describes a general, habitual fact, so subo (present indicative) is correct, not suba.

In Spanish, the subject pronoun (yo, tú, él…) is usually omitted because the verb ending already shows the subject.

• subo clearly indicates I (first person singular), so yo is not needed.
• You only add yo for emphasis or contrast:
  • Cuando yo subo las escaleras rápido, me duele la espalda, pero a mi hermano no.
    = When I go up the stairs quickly, my back hurts, but my brother’s doesn’t.

In neutral sentences like yours, dropping yo is the most natural option.

Both la escalera and las escaleras are used in Spain, but with a nuance:

• la escalera = the staircase as a single structure
  
  • La escalera de mi edificio es muy empinada.
    = The staircase in my building is very steep.
• las escaleras = the stairs (seen as a set of steps), more like English stairs
  
  • Subo las escaleras al tercer piso.
    = I go up the stairs to the third floor.

In everyday speech, especially for using the stairs, people very often say las escaleras, just like English the stairs is plural.

The verb subir can work in two main ways:

1. Transitive (with a direct object)

   • subir algo = go up / climb something
     
     • Subo las escaleras. = I go up the stairs.
     • Subo la montaña. = I climb the mountain.

2. Intransitive with a preposition

   • subir por / subir a
     
     • Subo por las escaleras. = I go up via the stairs.
     • Subo al tercer piso. = I go up to the third floor.

In your sentence, subo las escaleras uses subir transitively, which is perfectly standard and very common. You could say subo por las escaleras, but the meaning and feel are almost the same here; subo las escaleras is shorter and very natural.

In Spanish, many adjectives are also used as adverbs, especially rápido:

• As an adjective:
  • Un coche rápido. = A fast car.
• As an adverb:
  • Subo rápido. = I go up quickly.

Rápidamente is also correct and a bit more formal or neutral:

• Cuando subo las escaleras rápidamente, me duele la espalda.

In everyday spoken Spanish from Spain, rápido as an adverb is more common than rápidamente. Both mean quickly here.

Yes, Spanish allows some flexibility with adverbs like rápido, and all these are natural:

• Cuando subo las escaleras rápido, me duele la espalda.
• Cuando subo rápido las escaleras, me duele la espalda.
• Cuando subo las escaleras muy rápido, me duele la espalda.

Differences are very slight and mostly about rhythm and emphasis. You generally put rápido:

• After the verb: subo rápido
• After the object: subo las escaleras rápido

Both are perfectly fine in Spain.

The verb doler works differently from English to hurt:

• In Spanish, the thing that hurts is the grammatical subject, and the person who feels the pain is an indirect object.

So:

• Me duele la espalda.
  
  • Subject: la espalda (3rd person singular)
  • Verb: duele (3rd person singular, agrees with espalda)
  • Indirect object: me (to me)

Literally: The back hurts me.

Compare:

• Me duelen las piernas.
  
  • Subject: las piernas (plural)
  • Verb: duelen (plural)
  • Meaning: My legs hurt.

You cannot say yo duelo for this meaning. Yo duelo would mean something like I cause pain (very unusual and almost never said).

Tengo dolor en la espalda is grammatically correct, but sounds more formal/medical. In everyday speech, me duele la espalda is the default.

With body parts and clothing, Spanish usually uses the definite article (el, la, los, las) instead of a possessive (mi, tu, su) when it is clear whose body part we are talking about.

The combination is:

• Indirect object pronoun (me, te, le, nos, os, les)
• Definite article + body part

Examples:

• Me duele la espalda. = My back hurts.
• Me lavo las manos. = I wash my hands.
• Te cortas el pelo. = You cut your hair.

We already know it is your back from me, so mi espalda would sound redundant here. Mi espalda is possible in other contexts, e.g.:

• Esta es mi espalda en la radiografía.
  = This is my back in the X‑ray.

Yes, that is another natural option:

• Al subir las escaleras rápido, me duele la espalda.

Al + infinitive roughly means when / on / upon doing something. Nuance:

• Cuando subo las escaleras rápido...
  Focus on repeated, habitual situation.
• Al subir las escaleras rápido...
  Slightly more neutral/abstract; often used in explanations, instructions, or written language.

Both are correct in Spain; your original version is a very natural everyday way to say it.

In Spanish, the comma here is optional but recommended:

• Cuando subo las escaleras rápido, me duele la espalda.

You usually:

• Put a comma after a dependent clause placed at the beginning:
  • Cuando tengo tiempo, leo.
  • Si llueve, no salgo.

If you invert the order, there is normally no comma:

• Me duele la espalda cuando subo las escaleras rápido.

So your punctuation is standard and good.

The accent on cuándo depends on its function:

• cuándo (with accent) = interrogative or exclamative (direct or indirect):

  • ¿Cuándo subes las escaleras?
  • No sé cuándo subes las escaleras.

• cuando (without accent) = conjunction meaning when, introducing a clause like in your sentence:

  • Cuando subo las escaleras rápido, me duele la espalda.

Here it is just a conjunction, not a question word, so it does not take an accent.

You could say:

• Cuando estoy subiendo las escaleras rápido, me duele la espalda.

However:

• In Spanish, the simple present is more commonly used for:
  • General truths
  • Habits
  • Repeated situations

So for a general statement about what usually happens, cuando subo las escaleras rápido sounds more natural than cuando estoy subiendo…, which feels more like describing a specific ongoing event.

For your intended meaning (a habitual situation), cuando subo is the best choice.