How we cite our quotes: (Line)
Quote #1
Behold her, single in the field,
Yon solitary Highland Lass!
Reaping and singing by herself; (1-3)
The speaker directs our attention to the wonderful sight of the woman singing and working all by herself. The woman is the speaker's double: a fellow, solitary poet all alone out in the middle of nowhere.
Quote #2
Alone she cuts and binds the grain,
And sings a melancholy strain; (5-6)
The rhyme on "grain" and "strain" is important. Art and labor—song ("strain") and farming ("grain")—go together. Maybe the woman's beautiful art only exists when she is farming. Maybe true art can only be produced when some type of labor is happening (like farming).
Quote #3
O listen! for the Vale profound
Is overflowing with the sound. (7-8)
It really seems like the speaker has found himself in some kind of natural amphitheater. The "vale" is like an echo chamber that perfectly reflects the woman's song. If the speaker were from Colorado, he would say the whole scene reminds him of this.