Beyond tone, a poet's attitude toward his or her subject reveals to us a poem's theme.
The Theme of a poem it's its main idea or in general, terms what the author is trying to convey. Which involves the personal attitude of the author.
Structure in a poem is how it is formed, how stanzas are constituted.
Diction in a poem is the linguistic style that is used in it.
Subject is what the poem talks about, it can be confused with theme but the difference is that it only refers to the meaning of the words and their interaction together to the message.