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

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?