The title "The Solitary Reaper" makes me expect the poem to deal with a situation in country focused on a particular feeling, like loneliness.

The poem is divided in 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 are, because he cannot understand them, but only hears its notes.

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

 Every stanza has eight lines and a rhyme scheme ABABCCDD.

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

Level of MUSIC and SOUND (listen, music, sing, chaunt...)

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

Level of 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 very clear: the reaper's song was very wonderful and he wants to hear it for ever.