The poet saw a scotish girl absolutly alone in the field reaping and binding the crops . she was singing a so melodious song that poet thinks that she is singing well than a nightingale or a cuckoo bird. however poet was not able to identify what she was singing because she was singing in her mother tong (scotish laqnguage) . from the music he identified that it was a sad song . as poet continued his journey after sometime music stop to be heard . but then also it remained safe in poet's mind and heart . 
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William Wordsworth sees a reaper in the distance singing while reaping the crops. He is speechless and wonders what the reaper is singing about.....Why was the song so sad? He moves on and later this memory haunts him in the inward eye.