"Mariana" is a lyric poem written in iambic tetrameter. The rhyme scheme is ABABCDDCEFEF. The poem is composed of 7 12-line stanzas (similar to sonnets - 14 lines). The opening epigraph references Mariana from Shakespeare's play Measure for Measure. In the play, Mariana's lover has deserted her.
In the first stanza, the speaker describes the "moated grange" - an outlying farmhouse. There are signs of decay and these mirror Mariana's feelings. She feels abandoned. Each stanza ends with the same refrain (the final refrain is slightly different):
She only said, "My life is dreary,
He cometh not," she said;
She said, "I am aweary, aweary,
I would that I were dead!"
In the second stanza, the speaker describes how Mariana cried when the dew came in the evening, and she continued to cry before ("ere") the dews had dried. (The dew is analogous to her tears. As she cries, so does nature.) When night comes, all she can see is "the glooming flats." She could not appreciate the heavens (perhaps a beautiful sunset). Everywhere she looks, all she sees is gloom.
In stanza 3, she hears things which sound like her lover (Angelo): the "nightfowl crow" and the "oxen's low." This is a kind of pathetic fallacy: where inanimate things or animals echo the emotions of a character, in this case mocking her pain. This is of course a fallacy: Mariana perceives only the gloomy quality of things because she is so distraught.
Stanza 4 continues the description of gloomy surroundings. She feels surrounded, even imprisoned, by the "blackened waters" and the "clustered marish-mosses" (lumps of moss floating on the water). Mariana is symbolized by the poplar tree. No other tree "for leagues" (for a long distance) had to look upon ("to mark") such a dreary scene.
In stanza 5, she sees the shadow (of the poplar) swaying in the wind. The shadow of the poplar then falls on her bed (when the moon is low), as if she is in her own shadow, stuck in her own despair.
In stanza 6, every sound is affecting Mariana in a negative way. At this point, she may even be hallucinating, so affected by her loneliness.
In the final stanza, she is still plagued by every sound. Even the clock's ticking just indicates that she will have to face another day of loneliness. This is why she hates ("most she loathed") sunset. Sunset just precedes another lonely night followed by another lonely day. The poem is about the feeling of abandonment and how such a feeling an affect one's perception of other things, such as the home, nature, etc. The refrain is useful in showing how she feels this way over and over again, day after day. She is stuck in this feeling. She is so despondent that she sees death as the only way out; but she continues to wait. The continuing to wait is perhaps a small spark of hope in the midst of a vast amount of gloom.