In "The Road Not Taken," what does the sigh express?
Your original question had to be edited because it actually asked two questions. Multiple questions are not permitted on enotes, so please make sure in future that you only ask one question at a time.
The "sigh" that is referred to in the final stanza in this poem seems to encapsulate one of the central themes or messages of the poem as a whole. The poem is about decisions and choices and how we make choices in our lives that lead us on into new areas, but also how those choices can never be returned to. Just as the man has picked a path which leads him on "way to way," so that he can never go back and find out where the other path would take him, so our lives and where we are today are made up of a series of choices that have led us to where we are now. However, the "sigh" that the speaker refers to talks of the way that we are often haunted by the possibility of what could have happened at a major juncture of our life if we took a different decision. The haunting possibilities of this loss is what drives the poem to the "difference" that taking that path has led the speaker to.