One of the main themes is time. Time has been frozen in the scenes on the urn; however, the narrator ponders how the lovers will never actually kiss, for example, so they won't ever be able to realize thier love, etc. Another main theme is one of truth and beauty, but not in tne normal sense. The narrator seems to imply that:
..."truth is beauty, beauty truth" only applies in the place where all activity is stuck in one moment. We can certainly see the beauty: the lovers are in love, the music of the pipe is sweet, the trees are always full, and the people attending the sacrifice have the joy of anticipation. But where is the truth in all of this? It is a limited truth. (Enotes)
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The themes are eternity, foreverness. the unboundless love that the figures carved on the urn have makes our love seem to meek and can easily fade away...
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Also the theme love is scattered throughout Keats's poem. Keats says that the "Bold Lover, never, never canst though kiss" (17) the maiden even though he is so close to her. Keats also says though that the young man "yet do not grieve" (18) because "she cannot fade" (19) and will remain beautiful just the way she is forever. "For ever warm and still to be enjoy'd, For ever painting, and for ever young" (26-27). Keats is saying that that the man's love for the maiden will remain forever but the love between them is imperfect leaving the lover with "a burning forehead, and a parching tongue" (30).