In this excerpt from “Ode on a Grecian Urn” by John Keats, which parts reflect the theme that art is immortal?
Heard melodies are sweet, but those unheard
Are sweeter; therefore, ye soft pipes, play on;
Not to the sensual ear, but, more endear'd,
Pipe to the spirit ditties of no tone:
Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare;
Bold Lover, never, never canst thou kiss,
Though winning near the goal yet, do not grieve;
She cannot fade, though thou hast not thy bliss,
For ever wilt thou love, and she be fair!

Question

In this excerpt from “Ode on a Grecian Urn” by John Keats, which parts reflect the theme that art is immortal?
Heard melodies are sweet, but those unheard 
Are sweeter; therefore, ye soft pipes, play on;
Not to the sensual ear, but, more endear'd,
Pipe to the spirit ditties of no tone:
Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare;
Bold Lover, never, never canst thou kiss,
Though winning near the goal yet, do not grieve;
She cannot fade, though thou hast not thy bliss,
For ever wilt thou love, and she be fair!

Vocaloid · Answer

some important phrases:
"nor ever can those trees be bare" - the art on the urn does not suffer the effects of time as would real trees
"She cannot fade" - the woman on the urn does not age/die
"For ever wilt thou love, and she be fair!" the observer's love is temporary but the woman will stay beautiful and young