As the educator who previously answered this question mentioned, Neruda uses repetition—or a "refrain"—to create a sense of mournful suffering throughout the poem.
Additionally, Neruda uses personification to dramatize the theme of heartbreak. He personifies the night sky in the second and third stanza, giving it actions that are normally attributed to humans: "the blue stars shiver," "night wind revolves in the sky and sings." This picture of night becomes the backdrop against which Neruda recalls his love and how he "kiss[es] her again and again under the endless sky."
Simile is present later in the poem when Neruda comments that "the verse falls to the soul like dew to the pasture"; here, Neruda is comparing his lines of poetry being heard or read by a human soul to the condensation of water on grass in the morning.
Finally, Neruda uses hyperbole to emphasize the qualities he likes best about his former lover: namely, her "infinite eyes."
Repetition is the most obvious literary technique used by Pablo Neruda in his poem "Tonight I can Write." The line "Tonight I can write the saddest lines" is repeated thrice in the poem to emphatically foreground his sense of loss and sadness and to express his feelings of loneliness after the woman whom he loved has left him, "Tonight I can write the saddest lines/To think that I do not have her. To feel that I have lost her."
Throughout the entire poem he struggles to come to terms with the fact that the woman whom he loved deeply and passionately is no longer his but, "will be another's." This particular night reminds him of his passionate encounters with her in the past, only to reiterate the fact that she is no longer his: "Because through nights like this one I held her in my arms/my soul is not satisfied that it has lost her." And so, to highlight his sense of loss and loneliness he repeats the line "Tonight I can write the saddest lines" thrice.