The Spanish preposition 'por' meaning 'for' or 'by' comes from a merger of two Latin prepositions: 'per' meaning 'through' and 'pro' meaning 'before, for'. Over time, these two Latin prepositions blended together both in form and meaning in Spanish, resulting in the single preposition 'por' that took on various related meanings including movement through something, causation, and acting on behalf of something.

Some common Spanish words and phrases that use 'por' include 'por favor' (please), 'por qué' (why), and 'por eso' (therefore, for that reason). The preposition appears in countless Spanish expressions where it indicates reason, means, or movement.

English has many words derived from the same Latin roots. From Latin 'per' we get words like 'permeate' (to pass through), 'persist' (to continue through), and the prefix 'per-' as in 'perfect' or 'perform'. From Latin 'pro' we get words like 'provide', 'propose', and the prefix 'pro-' as in 'proclaim' or 'proceed'. Understanding these connections can help you remember that Spanish 'por' often involves the idea of movement through something or acting for a purpose.