Neruda's “Tonight I Can Write the Saddest Lines” describes a love story from the initial infatuation to the painful separation. This poem expresses the pain the poet feels after losing his beloved:

I can write the saddest poem of all tonight.
I loved her, and sometimes she loved me too.
My soul is lost without her

He uses a range of poetic devices to make his message felt even more strongly. He constantly juxtaposes images of closeness and love in the past with the present loneliness: this juxtaposition serves to heighten the pain he is feeling in the present because of the vivid contrasts between the two times. He also uses synecdoche when he talks about his "heart" and his "sight" seeking her: he obviously means that it's his person who longs for his lost love, but by using certain parts of him to speak for the whole, he manages to convey the manner in which the loss of his former love has affected him.

The poetic form of "Tonight I Can Write" by Pablo Neruda seems loosely based on the ghazal, an Arabic poetic form that combines a metrical pattern of couplets and a refrain with themes of loss and sorrow. Its distinguished Persian practitioners included Rumi and Hafiz; the form was popularized in western poetry by the German poet Goethe.

A poetic device that is part of the ghazal form is repetition. In the case of this poem, two poignant lines are repeated:

• Tonight I can write the saddest lines
• I loved her, and sometimes she loved me too

The repetition of these lines creates a tone of pathos and regret.

The next major poetic element is what is sometimes called the "pathetic fallacy" of using elements of the natural world as projections of the narrator's emotional state. The stars, dew, and wind are used as emblems of the narrator's internal feelings.

Another poetic element used in the poem is simile or explicit comparison. An example of a simile is found in the comparison of the poem to the dew in the line:

And the verse falls to the soul like dew to the pasture.