Expressing the Various Meanings of the Preposition "By" in Spanish
Means, Proximity, Mode... Let's Take a Look at What "By" Does in English and How to Say it in Spanish!
In a passive sentence, by is rendered as por, and marks the passive agent of an action. Thus, it shows means or instrumentality:
La ventana fue rota por los niños (The window was broken by the children).
El incendio fue apagado por las lluvias (The fire was put out by the heavy rain).
Me informaron de mi ascenso por correo electrónico (They told me of my promotion via/by e-mail).
Sometimes, by is used in such as way as to indicate an emotion or attitude, even a condition that has come into being. In such cases, de is used instead of por. Examples are worth a thousand words:
El Emperador Nerón fue muy odiado de los romanos (The Emperor Nero was hated by the Romans).
La actriz está rodeada de fotógrafos (The actress is surrounded by photographers).
When dealing with means of transportation and people are being transported, either por or en can be used. However, when people are not taking some form of transport, but rather things are being sent, then por is the only way to express by.
Mis amigos siempre viajan en avión pero yo voy siempre en tren (My friends always travel by plane but I always go by train).
Los cheques fueron enviados por correo de entrega inmediata (The checks were sent by special delivery mail).
However, when the English preposition by is used to express a deadline, in Spanish, the preposition para is used instead of por. This is one of the many distinctions between this troublesome pair of prepositions.
La tarea tiene que entregarse para mañana a más tardar (The homework must be handed in by tomorrow at the latest).
Hay que terminar el proyecto para el quince del mes (The project must be finished by the fifteenth of the month).
Sometimes, in English, the word by is omitted, but para must be used in Spanish:
¿Para cuando es la tarea? (When is the homeword due?)