Can you give an example of a literary device that these specific lines from "Romeo and Juliet" contain?
"Why art thou yet so fair? Shall I believe
That unsubstantial Death is amorous,
And that the lean abhorred monster keeps
Thee here in dark to be his paramour?"
These lines can be found within scene 3 of Act 5.
Please see the answer to a similar question posted nearby, as it deals with figurative language used in the lines just below.
Death is personified here. Personification gives human-like qualities to inanimate objects or abstract concepts. Death has no physical qualities until Shakespeare (through the voice of Romeo) makes him not only human but amorous. (If you don't know what "amorous" means, do look it up.) Then ask yourself, why is Romeo saying that Death is amorous? Why give "the lean abhorred monster" such a positive human quality?
You will see in the other answer how connotations are mentioned. This concept is extremely important when you are explaining figurative language in a short answer on a test, in an essay, or in a class discussion. Ask yourself why an author would use the image that an instance of figurative language creates rather than saying, "Juliet died."
Instead, Romeo uses romantic figurative language, saying that in Death is his rival for Juliet's love. Death has won Juliet's hand (in fact, her whole body). (We know that Juliet isn't really dead, but that's another literary device -- dramatic irony.) Why would Romeo make the image of Death so pleasant?
Also, note that Romeo uses contradictory images of positive, romantic ideas (amorous and paramour) while using negative images ("dark" and "lean abhorred monster"). Why would he say black and then say white; why say yes and then say no? Could it be that Romeo is overwhelmed with conflicting feelings? When you see someone you love passionately dead before you, and s/he is so very beautiful, is it possible that he is overwhelmed by both love (understanding how Death itself could fall in love with Juliet) and overwhelmed by hatred (despising Death for killing off his love)?
Remember when you write your explanation to really explore why Shakespeare uses such images, and who he has speaking such language. That will help you understand why he didn't say it more simply and directly ("Hi, I'm Romeo, and I'm really in love with Juliet and upset she died.") Wouldn't that be one boring play?