pretender vs fingir
False friend alert. 'Pretend' in English is fingir. Pretender means to intend or aim to do something.
fingir
To pretend, to fake, to put on an act.
Fingió estar dormido.
He pretended to be asleep.
Fingir + infinitive = to pretend to do or be.
No finjas que no lo sabías.
Don't pretend you didn't know.
Fingir que + clause. Subjunctive in the negative command.
Fingieron sorpresa.
They feigned surprise.
Fingir + noun = to fake an emotion.
pretender
To intend, aim or aspire to something, or to claim.
Pretendo terminar hoy.
I intend to finish today.
A plan, with some ambition behind it. Pretender + infinitive.
¿Qué pretendes con eso?
What are you trying to achieve by that?
Often suspicious in tone — what's your game?
Pretende que le crea.
He expects me to believe him.
Pretender que + subjunctive = to expect or demand.
Tricky bit
'Pretendo ser médico' means 'I aim to be a doctor', not 'I'm pretending to be one'. Say 'finjo ser médico' for the pretence.