Final answer:
The line from "A Hymn to the Evening" that contains a metaphor is "Soon as the sun forsook the eastern main".
Step-by-step explanation:
The line from "A Hymn to the Evening" that contains a metaphor is: "Soon as the sun forsook the eastern main". A metaphor is a figure of speech in which a word or phrase is used to describe something by suggesting a resemblance between the two. In this line, the setting of the sun is compared to the act of forsaking, creating a metaphorical image of the sun leaving the eastern main.
Learn more about Metaphors in poetry