Scansion is a poem's rhythm. Application is a word's appropriateness, and symbolism reinforces the word's relationship to a work's overall meaning. These elements are unified, and should be studied together. For instance, Shakespeare's Juliet speaks of a "rose by any other name." Shakespeare chooses "rose" for scansion; as a single syllable it fits into his iambic pentameter rhythm. He chooses it for appropriateness, as a romantic token. He chooses it for symbolism: Juliet is in conflict; her "rose," Romeo, has both a lovely scent and troublesome thorns.
Diction shows a poet's attitude towards a theme. Emily Dickinson's Poem 1260 has a remarkable final line -- "He will refund us finally our confiscated Gods" -- striking due to Dickinson's choice of "refund" and "confiscated." The use of diction here is to convey the poet's tone. "He" is God, and the Gods he has "confiscated" are our earthly loves, taken by death. The poet's diction thus makes death into a business transaction, trading life for the return of loved ones, and Dickinson's agnostic attitude toward belief in the afterlife is made clear.
Diction conveys connotation: both the reader's feelings about a poem and the feelings the poet experiences. Keats wrote "A Living Hand" when his death was near; he uses wintry phrases such as "icy silence of the tomb" and "chill thy dreaming nights," where his word choices -- icy, tomb, chill, nights -- reinforce death's coldness. Wordsworth, who had no such mortal fears, chooses, in his work "A Slumber," phrases such as "earth's diurnal course with rocks, stones and trees," all words that convey acceptance of death as a natural phenomenon.
The most enjoyable lessons examine diction for unexpected effects: E.E. Cummings "[In Just-]" remarks that a "balloonman whistles far and wee." "Wee," an adjective instead of the adverb, conveys size as well as distance. Cummings' masterpiece for unusual diction is "anyone lived," which speaks of a "pretty how town with up so floating many bells down," a line you can spend an entire class period analyzing for its diction effects.