How could I turn the poem "An Old Lesson from the Fields" by Archibald Lampman into a 300-400 word story?

The good news is, 300 words is not very long, so your story can be very concise and simple. This is a beautiful poem you have to work with – a speaker attuned to the natural world, watching the day wane and noticing the subtle changes that come over the earth and the sky “from noon til eve.”  This is a good place to start – with a person outside all afternoon. Why would he or she be outside? He or she could be hiking, could be working in the fields, could be taking a walk through a pastoral town. And who is your main character? The poem is in first person; you could adopt this same perspective for your story or you could give the speaker a name and tell it from a third person point of view, if you wished.

So let’s say you decide to go with an old man as your narrator; you could use the day fading from noon to evening as a metaphor for his life fading from youth to old age. And in this state, as the poem asserts, he could believe that his “soul was for the most part dead,” when compared to all the elements of nature, all those living things whose life's purpose is only to survive – their entire being is dedicated to living, pure and simple, whereas for our old man, there have been cares over the years to pull him away from simple living. These same cares pull us all away – school, job, home, family, bills, insurance, having children, sending those children to school and meeting all their basic needs, exceeding their basic needs, retirement, and above it all, striving to achieve happiness and contentment. Trees care not for any of these things, and our old man, looking back on a life of choice and responsibility, laments the fact that he could not live 100% according to instinct like the trees and the “tall lilies, of the wind-vexed field.”

Here we have an old man who is weary of life and who feels unfulfilled by it. He is surrounded by nature and, looking upon it, wishes his own life were or had been simpler, that he had been pulled in one single direction, rather than dozens, perhaps hundreds, over the course of it. 

You could have the entire story be one long thought for your main character; you could have him speaking to himself, or even addressing the various images he evokes in the poem – there are instances of this in the second verse.  

This sort of creative assignment cannot be done for you – you must find the words to do it yourself; find your own voice and have your story be an echo of it.  The poem is your guide; for your character’s thoughts, all you have to do is paraphrase.  For the rest, setting a scene should be fairly easy.  Good luck!