If a picture is worth a thousand words, a poem is worth ten-thousand. Poetry constructs an image and illustrates underlying ideas that the writer wants to convey. A poem has many aspects that make it what it is, the subject - or what the poem talks about on the surface, the theme - the underlying message expressed, and the tone - or the overall feel of the poem, are attributes that most poetry is full of.
The subject of a poem is what the poem talks about externally. In Stopping by Woods on a Snowy Evening, the first stanza talks about a man and his horse that are traveling as he comes upon some woods. He dismounts and takes a short stroll while admiring the beauty of it all. In the second and third verses, the horse acts confused as to why they are stopping there. During the final stanza the man expresses his awe in the splendor of the woods, but he realizes that he can not stay here forever, as he has things to do.
Loveliest of Trees talks about the beauty of the cherry tree. In the first verse Houseman details the magnificence of the cherry tree when it is blooming. The second canto is about the time in his life running out. In the last part of the poem he says that his time left alive is not long enough to admire the cherry trees. In Mending Wall there are two people who are walking along a fence to fix it in areas that are broken. The narrator doesn't really like having a wall but his neighbor insists on it, refusing to go against what his father has always said, "Good fences make good neighbors."
Themes in poetry are the underlying messages normally...