The title "The Solitary Reaper" makes me expect the poem to deal with a situation set in the country and  focused on a character's particular feeling of loneliness.

The poem is divided into  four stanzas. In the first one a girl is introduced. She is singing a melancholic song while she is reaping the crop.

In the second one her song is compared to a nightingale's.

In the third one the poet is wondering what the words of the song may be like, because he cannot understand them, but he can only hear its notes.

In the last stanza he concludes saying that it was such a wonderful song that he cannot forget it. 

Every stanza has eight lines and the rhyme scheme is ABABCCDD.

As far as the semantic level is concerned, we can find three important semantic fields:

MUSIC

SOUND (listen, music, sing, chaunt...)

NATURE (Arabian sands, Cuckoo-bird, Highland Lass...)

FEELINGS: (Solitary, melancholy, unhappy, sorrow, ...)

The prevailing theme is the theme of feelings, because the reader can imagine well what the poet remembers to have lived.The metaphor of the nightingale is clear: the reaper's song was so wonderful and he wants to hear it for ever.