write. Neruda conflates the "night" with the "night sky" or the "endless sky"—these can be understood as roughly equivalent in meaning. His example of the "saddest line" combines these elements (the night, the night sky, distance, coldness):

The night is starry
and the stars are blue and shiver in the distance.

These ideas become a reference point for understanding his feelings about his lover.

Take, for instance, the juxtaposition of the "sad line" "The night wind revolves in the sky and sings" with "I loved her, and sometimes she loved me too." The connection is emotive and suggestive, rather than explicit. The hidden nature of the night wind, unseen but heard, can be seen as a kind of natural counterpart to the inconsistent and ephemeral nature of his love—she loved him "sometimes," suggesting perhaps her affection varied like the wind, or that the incompleteness of their love was like hidden and forlorn, like the wind.

There is a similar pairing a few lines later, when Neruda juxtaposes these lines:

I kissed her again and again under the endless sky
She loved me, sometimes I loved her too.

The endless sky, implicitly a night sky, is contrasted with the inconsistency of their love, this time on his part: his kisses (perhaps endless, like the sky?) are nevertheless only an indication that "sometimes" he loved her.

This kind of comparison is more fully realizes in the line "To hear the immense night, still more immense without her." Neruda connects the night, "immense" because it covers the world in darkness, with the absence of his lover. His longing for her, which includes his memory of their incompleteness, only adds to the darkness of the night.

In the line "I kissed her again and again under the endless sky," the "endless sky" symbolizes the love that the speaker felt for this woman when he kissed her "again and again." At this moment, with this woman, the speaker's love felt boundless. Describing his loss of this woman, the speaker uses the symbol of "the immense night"—which represents a heavy, oppressive darkness—and also perhaps a sense of disorientation.

Later in the poem, the speaker says that his "heart looks for her." Here the speaker's heart acts as a symbol for his love. It is also personified as actively looking for the woman, and the implication is that his heart is sad and incomplete: wandering in vain to find its lost love.

The woman's eyes are referenced several times throughout the poem—in one instance as "her great still eyes" and on another as "Her infinite eyes." Eyes are often referred to as the "windows to the soul," so the woman's eyes here could symbolize the soul (or the essence) of the woman that the speaker fell so much in love with. The eyes are described as "great" and "infinite," suggesting that the woman had a large, open soul.

The stars described in the poem as "blue and shiver[ing] in the distance" symbolize the woman herself. She was—and perhaps still is, to the speaker—a light in darkness. She is also, like the stars, unreachable. The stars are an appropriate symbol to represent the woman because they are at once beautiful and mysterious—visible, but out of reach.

Neruda uses night and day as a major symbolic elements in "Tonight I Can Write." 

As the poem progresses, the night becomes increasingly associated with the period of time when the narrator feels he loved a woman - the woman he writes about in the poem. (We might point out that the woman seems to be nearly symbolic herself and may represent an aspect of life or youth.)

We might take the night to symbolize love or the emotional coherence of the narrator's love. It is this that has been "shattered" and which feels "still more immense without her" as the narrator reflects on the scope of the love he once had for the woman. 

Having lost the woman, the poet writes the lines of the poem and the "verse falls to the soul like dew to the pasture," suggesting that his sadness is connected to morning dew, daybreak or the night's end. 

The poem's retrospection points to an idea that perhaps more than love has been lost by the narrator. Perhaps a way of being for the narrator has been lost as well.

The same night whitening the same trees.
We, of that time, are no longer the same.

Where the narrator once was able to love the woman while she only sometimes loved him, as he narrates the poem he feels that now he no longer loves her as he once did. His former fervor and dedication have turned to diffidence.

He now may love her, but not with certainty or clarity. The night has passed where that love lived/existed fully.

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.

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."