Cuando encuentres una paciente que sea sincera, aprenderás mucho de su historia.

to be

Breakdown of Cuando encuentres una paciente que sea sincera, aprenderás mucho de su historia.

• una: a
• tú: you
• aprender: to learn

Questions & Answers about Cuando encuentres una paciente que sea sincera, aprenderás mucho de su historia.

Why is it cuando encuentres and not cuando encuentras?

In Spanish, cuando + subjunctive is used when you are talking about a future event that is uncertain or has not happened yet.

• Cuando encuentres una paciente… = When you (eventually / someday) find a patient… (not yet happened, future / hypothetical)
• Cuando encuentras una paciente… would normally be used for something that happens regularly or habitually, like:
  • Cuando encuentras una paciente sincera, siempre aprendes mucho.
    When(ever) you find a sincere patient, you always learn a lot.

In your sentence, the idea is a specific future occasion, not a general habit, so Spanish prefers the subjunctive encuentres, not the indicative encuentras.

Why is it que sea sincera and not que es sincera?

The subjunctive sea is used because the patient is not a specific, known individual; it is an indefinite/unknown person that may or may not exist.

• una paciente que sea sincera
  literally: a patient who may be / is hopefully sincere
  The sincerity is part of what you are looking for, not a known fact.

Compare:

• Conozco a una paciente que es sincera.
  I know a patient who is sincere. (specific, real patient → es, indicative)

• Busco una paciente que sea sincera.
  I am looking for a patient who is sincere. (you don’t have her yet → sea, subjunctive)

Your sentence has the same grammar as busco una paciente que sea sincera, so it uses sea.