What were the conjectures of Wordsworth as he listened to the song and wondered about?

The poem is made up of four stanzas. In the first stanza, Wordsworth sets the scene for the readers. He asks us to observe the Highland girl busily reaping the ripe grain  and singing to herself. He asks us to pause and listen to the song which fills the entire valley,or quietly leave the place without disturbing her.

In the second stanza, Wordsworth tells us that  her beautiful song was more  refreshing than  the melodious song of the nightingale which welcomed the weary travellers as soon as they arrived at an oasis and that her song was more pleasing than the cuckoo's song which signalled the end of the harsh winter season and the beginning of spring.

Since Wordsworth could not understand Gaelic, the language of the reaper, he impatiently asks whether someone could tell him what she was singing about. By doing so he sparks our imagination as to what she could be singing about.

Will no one tell me what she sings?--

Perhaps the plaintive numbers flow

For old, unhappy, far-off things,

And battles long ago:

Or is it some more humble lay,

Familiar matter of to-day?

Some natural sorrow, loss, or pain,

That has been, and may be again?"

Soon,Wordsworth leaves the scene concluding  that although he could not understand what she was singing about nevertheless he could always remember the melodious tune of her song:"The music in my heart I bore/Long after it was heard no more."

Wordsworth wonders whether she is singing about the past - about some sorrowful incident of the past, like a defeat in a battle OR about some unhappy incident in the present which may be repeated again in the future. The important thing to remember is that whether it is the past, the present or the future Wordsworth is convinced that what she is thinking about is sad and sorrowful which is echoed in the melancholic tone of her melody.

The words 'single' 'solitary' and 'alone' have been foregrounded. 'Single' implies that she is the only person in the valley; 'solitary' hints at the melancholy mood of the poem and 'alone' refers to the fact that there is no one to assist her in her labours.