Robert Frost's poem has the structure of a lyric poem. If you think about it, this poem could be put to some music and be sung, just like song lyrics.
Compound subjects are subjects joined by words like and. A subject is a person, place, or thing. A noun basically.
I would like to visit Rome and Athens, because it's very beautiful. Rome and Athens would be underlined.
It is my dream to visit the Arizona, more specifically, the Grand Canyon.
Arizona and Grand Canyon would be underlined .