Por vs Para
Both por and para often translate to English 'for' — that's where the confusion starts. Simple rule: para looks forward (purpose, destination, deadline), por looks backward or inward (cause, motion through, exchange). You can't build a real sentence in Spanish without this pair.
PARA — purpose and recipient
Purpose / goal: Estudio para hablar español (I study so I can speak Spanish). Recipient: Este libro es para ti (this book is for you). Deadline: Para mañana (for tomorrow / by tomorrow). Direction: Salgo para Madrid (I'm leaving for Madrid). Opinion: Para mí, es bueno (in my opinion, it's good).
POR — cause and exchange
Cause / reason: Lo hago por ti (I'm doing it because of you / for your sake). Duration: Vivo aquí por dos años (I've lived here for two years). Movement through: Camino por el parque (I walk through the park). Exchange / price: Compro pan por dos euros (I buy bread for 2 euros). Time of day: por la mañana, por la tarde, por la noche.
Trap: 'for you' (the recipient) = para
When it's a gift, a thank-you, or a message — that's para: Una carta para mamá, flores para ti, un regalo para Ana. If you'd say 'this is for so-and-so' in English, it's para.
Trap: 'because of / for the sake of' = por
If you'd say 'because of', 'on behalf of', 'thanks to', or 'for' (in the sense of motivation) — that's por. Llego tarde por el tráfico (I'm late because of traffic). Lo hago por amor (I do it for love / out of love). Gracias por la carta (thanks for the letter).
Set expressions
por favor (please), por supuesto (of course), por ejemplo (for example), por fin (finally), por eso (that's why), por aquí (around here). para siempre (forever), para nada (not at all). Easier to memorize as units.
