I like to think some boy's been swinging them (Line 3)
The image of the birch trees under snow and ice conjures up some vivid memories for the speaker. When he sees birches he immediately thinks back to the days when he would swing on their trunks and wishes that some other boy were enjoying the trees as he had.
Such heaps of broken glass to sweep away (Line 12)
It's interesting that the speaker moves quickly from the realm of the natural world to that of human beings. There seems to be a tension between Nature and Man in "Birches," and here we see that he can negotiate easily between the two. Do you think that the speaker prefers one sphere over the over?
Like girls on hands and knees that throw their hair (Line 19)
It almost seems as if the speaker is noting that boys and girls appreciate nature in different ways. Does he imply that boys are more active, or is he merely stating that there is a difference? Do you think that there is a gender difference, and if so, what is it?
One by one he subdued his father's trees (Line 28)
Does it seem like the father or the son has dominion over the natural world? Or is this meant to reveal that the weight of the boy has physical consequences for the straightness of the trees? How would you interpret this line?
And life is too much like a pathless wood (Line 44)
One interpretation of this line is that the speaker has an antagonistic attitude towards nature. What other interpretations can you come up with? Is he talking about direction in life, or a lack thereof? Or is he getting at something more fundamental to human nature?