There are different emotional experiences that Hardy goes through in "The Darkling Thrush." One example of how he communicates his feelings occurs when he describes the scene of bleakness that is in front of him. Throughout the poem, Hardy is able to establish that he feels a great deal of sadness about what he sees and the world in which he lives. These feelings are established in the poem's opening stanza. Terms like "spectre-gray" frost and "the weakening eye of day" communicate a feeling of decay. Hardy experiences this internally and uses language to describe the world he sees as part of his feelings and emotional experience. Hardy feels alone in what he is experiencing, separate from the human beings who are inside and "had sought their household fires." In the first stanza, Hardy uses a description of the setting as means to communicate his emotional experience.
Hardy is able to use the natural world and his experiences within it as ways to communicate his feelings, as well. For example, the use of "death-lament" to describe the wind is a way to communicate his feelings about what he sees in front of him. He also is able to capture this feeling of isolation by describing the seed as "shrunken hard and dry" and comparing himself to this experience. In being able to identify with the natural world around him, Hardy is able to again externalize the internal experience. Doing so helps to communicate his emotions of loss, displacement, and his sense of pain at being in the world.
Finally, I think that Hardy is able to communicate his feelings about being in the world when he hears the song of the thrush. He uses contrasting language such as "bleak twigs" and "joy illimited." Hardy uses the physical description of the thrush as "aged, frail, gaunt, and small" to contrast with the "full hearted evensong." These contrasts help to communicate Hardy's own feelings of joy and despair that exist simultaneously. The result of such a contrast is that Hardy is "unaware" and puzzled at the condition of consciousness. Hardy is able to describe these confusing feelings by bringing out the contrasts that exist in the world. There is coldness and despair, but there can be hope and beauty amidst such ruins. Such a contrast is utilized to illuminate the feelings that Hardy experiences.