What is the difference between 'por' and 'para'

'Por' and 'Para' often cause confusion because both can be translated in English as 'for'. The best way I have found to understand this is using the example of presents. Consider the phrase 'I a bought this present for Juan'. In one sense, the Juan is the target of the present, its recipient, maybe it was Juan's birthday and you bought him the present. If we look at it another way, however, you could say that Juan was actually the person on whose behalf you bought the present. Say you were going to Maria's party and Juan had forgotten to buy her a present, then you bought the present for Juan in the sense you bought it on his behalf. This unravels the key difference between 'para' and 'por'. In first example, the Spanish use 'para' to indicate the destination of the gift-giving: 'compré un regalo para Juan'. In the second, Spanish uses 'por' to indicate the origin or cause of the gift. This key distinction can then be applied to other uses of por/para e.g. passive tense.