Why does the speaker guess the song's meaning in "The Solitary Reaper"?

In "The Solitary Reaper," the speaker stops to hear the song of a lone young woman reaping in a field in a valley not too far away from him. The song she sings arrests him: he says it is more beautiful than the song of a nightingale or a cuckoo. He listens and can hear that the tune is "plaintive," or sad and melancholy, but he can't make out the words.

While it is never fully explained why he can't discern the words, the implication is that he is too far away—or that she sings in a Scottish dialect that he can't understand. Therefore, he questions what she is singing about, wondering if it is about events from long ago or everyday but sad events from the present

The speaker's inability to make out the words emphasizes the young woman's merger with nature. Just as humans can't make out "words" in a bird's song, so the speaker is compelled to listen to a series of beautiful notes or sounds without assigning them a specific meaning. This adds to the mystery and beauty of her song.

The speaker moves on finally and starts to hike again, but the memory of the song lingers long after he can no longer hear it.

Why did William Wordsworth stop to listen to the Solitary Reaper's song?

Although it is written in a first person point of view, the speaker of the poem is not necessarily William Wordsworth. The speaker seems to be a traveller who is about to walk up a hill when they hear the reaper's song. The woman reaping in the fields is singing "a melancholy strain" in another language, but the speaker says it makes them feel welcome. The speaker stops to listen more closely to the reaper's song, as he cannot determine what she is singing about. The speaker wonders if it is about the past, issues in modern times, or a more timeless feeling, like sorrow, pain, or loss. Regardless of what the reaper's seemingly never-ending song is about, it makes the speaker feel connected to the reaper and they are moved by her song. The speaker is acknowledging how much music can unite people, even if they are from very different backgrounds and do not speak the same language. The speaker holds the music in his or her heart "long after it was heard no more."

How does Wordsworth create rhythm and musicality in "The Solitary Reaper"?

In "The Solitary Reaper," Wordsworth highlights his focus on music by writing the poem in a rhythmic, lyrical style. He accomplishes this lyrical effect in a couple of ways. For one thing, the poem is written in iambic tetrameter, which is a tighter composition than the more common iambic pentameter and contains only four feet per line. This tight construction ensures a quicker, more rhythmic transition between lines and results in a lyrical effect. Additionally, Wordsworth's verse is filled with alliteration. Take, for instance, the following lines:

Behold her, single in the field, 
Yon solitary Highland Lass! 
Reaping and singing by herself; 
Stop here, or gently pass! (1-4)

What were Wordsworth's conjectures as he listened to the song in "The Solitary Reaper"?

As the speaker in Wordsworth's poem "The Solitary Reaper" hears the song, there is much that allows for conjecture and a sense of wonderment to emerge.  The speaker does not know the what the song means, but this lack of literal comprehension allows for a greater development of symbolic understanding of the song which is put forth by a series of images.  One such image is the idea of that the melody of the song heard in the present can transcend time and move a listener to it into the past.  The image of the song being as perfectly natural as the song of a "nightingale" or "cuckoo" resonates in the speaker's mind.  The speaker becomes transfixed with the song and conjectures the setting of "Arabian sands," implying a world of mystique and unknown.  Additionally, the song can be heard as far as "the Hebrides," a strip of islands far removed from civilization.  Another conjecture is that the song is song in recognition of past battles and conflicts ("battles long ago") or some type of emotional hurt that cannot be expressed in words, but is articulated through this song (lines 21-25).