How does John Keats's Poem "Ode on a Grecian Urn" illustrate Romanticism?

For the Romantics, poetry was the highest form of literature because it most easily expressed the concept of the "sublime" (a thrilling emotional experience that combines awe, magnificence, and horror).  Romantic literature also emphasizes the dream, or inner reality of the individual.  To express the inner reality of the consciousness of the individual, imagery and symbolism was often employed.

In John Keats's "Ode to a Grecian Urn," the poet clearly expresses the sublime as he contemplates the beautiful urn with its painted lovers and expresses a nostalgia for the Classical era of the past:

What leaf-fringed legend haunts about thy shape/Of deities or mortals, or of both,/In Tempe or the dales of Arcady?/ What men or gods are these?  What maidens loth?

Here, too, Keats expresses an awe for the beauty of the musicians and the two lovers on the urn, images that suggest the timeless beauty of art.  There is a perfection in this art because the musician will always play, the lovers always love.  The trans-formative element of Romantic poetry is also present here as the poet perceives how there is a truth to beauty that goes beyond what is.  And, here Keats expresses his theme:  There is mortality to all things, including beauty.  In lines 11-14, for instance, Keats draws the distinction between ideal nature of art and the flawed and fleeting nature of life:

Heard melodies are sweet, but those unheard/ are sweeter; therefore, ye soft pipes, play on;

Thus, there is a tension between the temporal(men) and the eternal (god).  Employing the symbols of "trees," for Nature, "song" for Art, and "Bold Lover" for procreation, Keats explains this tension between what is on the urn with what is real.  For the urn, "truth is beauty, beauty truth," since it is frozen in time.  But, for the poet who knows that beauty does not last, the truth is not restricted to the images on the urn; for the poet, teased out of thought, realizes that the lovers will never consummate their love, but will remain in only their moment.   And, here the poem becomes more melancholy, a characteristic often found in Romanticism.

Stressing the power of the imagination, John Keats's "Ode on a Grecian Urn" reflects the thinking of the Romantic in its reflections upon the Classical beauty of the ancient urn and its transcendental properties of transporting this imagination to understanding truths of Nature and Art and Humanity.