To say "by" [someone/something] we use the preposition "por" in Spanish.
Here are some examples:
Ese edificio diseñado por Gaudí es espectacular.That building designed by Gaudí is spectacular.
El ladrón, arrestado por la policía ayer, está en la cárcel.The thief, arrested by the police yesterday, is in prison.
Al final fui a la fiesta, animada por mi amiga Clara.In the end I went to the party, encouraged by my friend Clara.
Por is generally following an adjective in the form of a participle.
Here are more examples:
Las calles, iluminadas por las farolas, estaban solitarias.The streets, illuminated by the street-lamps, were deserted.
El gato, amenazado a menudo por el perro, se escondía en el jardín.The cat, often threatened by the dog, used to hide in the garden.
Jaime, aplaudido por todos, dejó la sala de conferencias.Jaime, applauded by everyone, left the conference room.
Mis hijos fueron al concierto persuadidos por sus amigosMy sons went to the concert [after being] persuaded by their friends.
Por is also used in passive sentences to indicate by whom the action was done. To learn about this see Forming the passive voice with ser + participle (+ por) (passive - simple tenses)
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