Cuando subo las escaleras rápido, me duele la espalda.

When I go up the stairs quickly, my back hurts.

Breakdown of Cuando subo las escaleras rápido, me duele la espalda.

• yo: I
• me: me
• cuando: when
• .: period (punctuation mark)

Questions & Answers about Cuando subo las escaleras rápido, me duele la espalda.

Why is it subo and not suba after cuando? I thought cuando often takes the subjunctive.

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.

Why don’t we say yo? Why is it just cuando subo instead of cuando yo subo?

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.