Answer: Although both detrás and atrás are adverbs that can be translated as "behind" and are often listed as synonyms, they tend to be used in different ways. Atrás tends to indicate motion backwards, while detrás tends to refer to place, but the distinction isn't always clear. Sometimes the choice of word is a matter of which "sounds better" rather than following some fixed rule.
That said, it is probably easier to explain these verbs by pointing out when you are most likely to hear them used. Detrás is most often used:
- As detrás de to form a prepositional phrase meaning "behind," "in back of" or "after." Está detrás de la casa. (It is behind the house.) Vinieron detrás de ella. (They came after she did.)
- In a figurative sense as detrás de to mean "behind." Condenó a los políticos detrás de las protestas. (He condemned the politicians behind the protests.) El cuento detrás del cuento. (The story behind the story.)
- As por detrás to form a phrase meaning "from behind." Se rieron de ella por detrás. (They laughed at her from behind her back.)
Atrás is most often used:
- By itself or as hacia atrás to mean "backwards." Fue atrás. (He went backwards.) Miró hacia atrás. (He looked backwards.)
- To mean "ago." Comí cinco días atrás. (I ate five days ago.)
- With dejar to mean "leave behind." Tim LaHaye escribió el libro Dejados atrás. (Tim LaHaye wrote the book Left Behind.)
- As an exclamation (¡atrás!) to mean "Get back!"
- In preference to detrás when accompanied by a comparative term such as más or menos. Otro edificio más atrás servía de oficina. (Another building further back served as an office.)