During his life Neruda was known as quite the womanizer at times, having multiple marriages. This poem is probably written about one of his wives or lovers that he once loved that he is trying to get over. At one point he says "my soul is not satisfied that it has lost her," however he states he no longer loves her. His soul is tormented, caught between two different feelings, one of missing his former love and one feeling of the need to move on.
The book also talks about how well he incorporates nature into his love poems. Tonight I Can Write... is no exception, comparing his situations to stars, nights, the sky and the trees. He lets the reader immerse them self into their own natural world feeling what he is feeling through natural forces.