How does the contemplation of a Grecian urn relate to the subject of the poem in "Ode on a Grecian Urn?

Throughout the poem the urn functions as a symbol of
eternity which is compared against the brief and transitory state of man's existence on
the planet. Note how the speaker of the poem refers to the urn in the first
stanza:

Thou
still unravished bride of quietness,

Thou foster child of
silence and slow time,

Sylvan
historian...

These images
have been carefully chosen to emphasise the way that the urn that is being contemplated
is undamaged by the ravages of time and how it preserves history. Thus it is that this
urn and the scenes that the speaker views on it triggers off a series of reflections
about beauty, art and mortality, that are expressed in the last stanza. However, there
is a crucial ambiguity that is created in this stanza that is never resolved. We are
left unsure as to whether the urn is being celebrated as a symbol of the eternal nature
or art and beauty, or whether it is used to make a critical comment on the limitations
of art and the need to turn to life rather than idealised expressions of beauty.
Consider the following quote:

readability="20">

When old age shall this generation
waste,

Thous shalt remain, in midst of other
woe

Than ours, a friend to man, to whom tho
say'st,

"Beauty is truth, truth beauty"--that is
all

Ye know on earth, and all ye need to
know.

Note how both the
positives and negatives of contemplating the urn as an ideal of art and beauty are
explored. On the one hand, such a contemplation gives us a sense of the the timeless
nature of art. On the other hand, this contemplation only exacerbates our awareness of
our own mortality and eventual death.

Thus you might find
it helpful to consider what the urn symbolises in the poem, and the author's ambivalent
attitude towards the symbolic meaning.