In "Ode on a Grecian Urn" how do you interpret lines 28-30?
It is always important to look at the context of the lines you need to focus on to see what they mean and to try and understand what clues the rest of the poem before and after can give in order to help us understand what is going on. These are the lines your question refers to:
All breathing human passion far above,
That leaves a heart high-sorrowful and cloyed,
A burning forehead, and a parching tongue.
This stanza indicates that Keats, through contemplating the urn that the poem is based on, has managed to briefly leave his own human world and enter into the world of eternal beauty that is symbolised through the urn. Note how the repetition of the word "happy" in this stanza reflects this. However, as Keats realises that he is not actually part of this world and that he is profoundly separated from such a world, the stanza ends by reflecting on the way in which the passion of the lovers on the urn is "far above," indicating that it exists in an abstract, rhetorical realm rather than in the real world where Keats himself dwells.
The heart of the speaker, in response to this realisation, is "high-sorrowful and cloy'd" because of the way that his physical feelings, his "burning forehead, and a parching tongue," acts as yet another reminder of the real world that he is part of. Although he has managed to enter the world of the urn and the eternal beauty that it represents, this has just been for a moment, and he is left frustrated by the unbridgable gap between his own world and the world that he wants to experience more of.