Both mean "To Be" — Here is the Infallible Rule:
SER (D.O.C.T.O.R.)
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Description (Soy alto)
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Occupation (Es músico)
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Characteristic (Son amables)
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Time / Date (Son las tres)
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Origin (Soy de Londres)
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Relationship (Ella es mi hermana)
ESTAR (P.L.A.C.E.)
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Position (Está sentado)
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Location (Estoy en casa)
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Action / -ing (Está comiendo)
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Condition (El café está frío)
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Emotion (Estamos felices)
Both mean "For" — How to Choose:
POR = Cause / Reason / Exchange
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Reason: Gracias por todo (Thanks for everything)
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Duration: Por dos horas (For two hours)
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Exchange: Pagué £20 por esto (I paid £20 for this)
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Means of transport: Por avión (By plane)
PARA = Goal / Recipient / Deadline
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Recipient: Esto es para ti (This is for you)
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Destination: Salgo para Madrid (Leaving for Madrid)
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Deadline: Para mañana (For tomorrow)
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In order to: Estudio para aprender (I study in order to learn)