We could identify several themes in Pablo Neruda's "Tonight I Can Write," but the poem is primarily a meditation on the nature of love, loss, and memory, as well as about how life experiences inspire the speaker's poems.
We learn gradually in the poem that the reason the speaker "can write" on this night is that he has lost his beloved. This loss has inspired him to reflect on their love and on the nature of love itself. He says that "love is so short, forgetting is so long" (28). While the love itself, the feeling and the relationship both, have ended, he will still remember it for much longer. He even immortalizes it through writing this poem.
Love and memory constantly intermingle in the poem, to the extent that remembering this beloved makes the speaker question whether he really has ceased to love her. Take these lines for example:
I no longer love her, that's certain, but how I loved her.
My voice tried to find the wind to touch her hearing.
Another's. She will be another's. As she was before my kisses.
Her voice, her bright body. Her infinite eyes.
I no longer love her, that's certain, but maybe I love her. (23–27)
This section begins with the speaker confidently stating that he "no longer love[s] her, that's certain." However, once he thinks about her some more, and imagines her as with someone else, he is less sure. Even after he repeats that he is "certain" he doesn't love her any more, he immediately says, "maybe I love her." The memory of her brings back some of the feelings he experienced when they were together.
Beyond the meditation on love, loss, and memory, the poem is also about the writing process and how poets are inspired to create their works. The speaker claims that on this night, he is able to "write the saddest lines," because he is reflecting on this former lover and his loss of her. The process of writing connects to the theme of memory as well, and again, possibly rekindles some of the speaker's love.