por

Por is a preposition present in Spanish and Portuguese that covers a wide range of relations. It can express cause or motive, duration, means or method, movement through or by, and exchange or price. It also marks the agent in passive constructions, as in “La carta fue escrita por María.” In both languages, por appears in numerous set phrases such as por favor (please), por supuesto (of course), por eso (therefore), por ejemplo (for example), and por ciento (percent).

Etymology and scope. Por derives from Latin pro, meaning for, in favor of, on behalf of, or through. Its versatility stems from shifting the relation it signals in a sentence, making it one of the most frequent prepositions in the two languages.

Spanish usage. Common functions include causa o motivo (lo hizo por ti), duración temporal (dos horas por día), medio o instrumento (viajé por tren), paso o atravieso (caminaron por la avenida), y precio o intercambio (vendido por cien euros). Por also marks the agent in the passive voice (fue escrito por García Márquez). Distinctions with para are crucial: por often signals motive, means, or approximate time, whereas para marks purpose, destination, or recipient.

Portuguese usage. Portuguese uses por similarly: duration (duas horas por dia), means (falou por telemóvel), path or through (passar por ali), and agent in passive voice (foi feito por João). The language additionally treats question forms and related words with four spellings: por que (why in questions), por quê (why at the end of a sentence), porque (because), and porquê (the reason). Fixed phrases such as por favor, por isso, and por cento are common.

Notes. The choice between por and para depends on the relation expressed—motive, means, or passage versus destination, purpose, or recipient. Por remains a versatile, often ambiguous preposition requiring attention to context.