Here's how it works:
* The central metaphor: The poem uses the image of "writing the saddest lines" as a metaphor for expressing profound sadness and heartbreak. It's not literal writing, but a representation of the poet's emotional state.
* Extended metaphor: The metaphor is extended throughout the poem, with various elements contributing to the overall feeling of sorrow, loss, and emptiness. For instance, the lines "My soul is not satisfied with having loved her" and "Love is so short, forgetting is so long" are further developments of the metaphorical "writing the saddest lines."
While metaphor is the dominant figure of speech, there are other devices used as well, such as:
* Personification: Giving human qualities to objects. For example, "Love is so short, forgetting is so long."
* Symbolism: Using objects to represent abstract concepts. For instance, the recurring image of the "empty" city and the "rain" can be seen as symbols of loneliness and the overwhelming nature of grief.
Ultimately, Neruda's poem uses a combination of literary devices to create a powerful and evocative expression of heartbreak and the bittersweet nature of love and loss.