One interesting element that may draw the attention of the reader is the use of the adjective "solitary".

The choice may anticipate a melancholic atmosphere or a sad one.

The speaking voice invites the passer-by to look at a girl who is working alone in the field.

While she's reaping she is also singing and the poet seems to be inviting the passer-by not to disturb her.

Her song is melancholic and seems to fill all the valley also creating an echo effect.

The poet compares the girl's song to that of a nightingale or of a cuckoo-bird, but the girl's one seems to be better. The reaper's voice is so magic that the poet says no other sound could be compared to it.

The girl's song causes strong emotions to the poet although he cannot understand what the matter of the song is about.

Some ideas about the possible content of the song are expressed by the writer in a series of hypothesis.

Finally the speaking voice reflects about the experience he has just taken before he walked away.

The girl's song was heard no more by the poet but he brought that striking music in his heart long after walking away.

The poem is arranged into four octaves in which descriptive and reflective parts alternate.

The speaking voice at first focuses his attention on a girl who is working and singing alone in the field, then provides the reader with a not detailed and quite implicit description of the country landscape.

The poet's thoughts and feelings derived from the girl's song are exposed in the second stanza.

The third one consists of a list of hypothesis about what could inspire the girl's melancholic way of singing.

The last stanza contains the conclusion about the brief experience of the poet.

The first two lines of the poem are characterized by a series of words which have open vowels. This element contributes to create a sensation of quiet, calm and may suggest the image of an opened and boundless landscape to the reader's mind. Also the enjambement at line eighth conveys the vastness of the landscape.

The last two lines of the first stanza contain the repetition of the vowels "o" and "u" which seem to actually reproduce the sound spreading in the valley.

At line thirteen,  the word "thrilling" is an onomatopoeia because it resembles to reproduce the girl's voice.

The rhyme scheme contributes to add musicality to the poem and it follows the ABABCCDD scheme with few exceptions.

The poet uses a lot of punctuation to give rhythm to the poem.

For example, he uses the exclamation mark at the end of lines two, four and seventeen to better express the strong involvement he has with the reaper.

The dasch used at line seventeen anticipates a list of hypothesis the poet makes and the dasch at line twenty-eight is used to separate the previous descriptive and objective  part from the following reflective part.

The main semantic fields are solitude and introspection (single, solitary, by herself, alone, melancholy, plaintive, unhappy, sorrow, loss, pain...), sound (singing, strain, listen, sound, chaunt, notes, voice, heard, music...), and the work in the fields (reaping, cuts, binds the grain, work, sickle...).

These semantic fields seem to suggest a strong connection between the girl's song and the nature landscape. The girl can find again a contact with herself staying in the nature and she can rediscover her deepest feelings she will express through her song.

At lines twelfth and sixth, there are two hyperboles  (among Arabian sands / among the farthest Hebrides) used by the poet to magnify the power of reaper's voice.

Also the verb tenses contribute to add meaning. The poem starts with an imperative (behold, stop) which suggests the poet's mood, while the verbs "reaping" and "singing" give a dynamic effect to the poem.

At first, there is the use of the simple present or present continuous because the poet is actually living his experience; then the writer uses the present perfect (has been) to refer to something which has just happened and may have its effect in the present. The  use of the modal "may" suggests a projection to the future.

In the end, the use of the simple past indicates that the poet's experience is already finished.