Vocabulary in Ode on a Grecian Urn
Brede is an interwoven pattern. Although Keats is using it literally to describe the art presented on the urn, notice how it also describes the tangled nature of the thoughts and philosophical questions the poem presents.
That is, the "happy love" portrayed on the urn proves itself to be far better than human love or "passion," which necessarily decays with time and becomes less beautiful and less pure as it goes. "Breathing" here may as well mean "mortal" or "temporary."
Loth meaning reluctant or unwilling, bashful maidens. From this description, we can assume that there's a scene depicted on the urn wherein several men or gods address a group of women, likely in a romantic overture. These women, though beautiful, shy away from their advances.
In ancient Greece, the pastoral referred to the ideal state of nature. In poetry, the pastoral is a type of poetry that glorifies that ideal nature. It's described here as being "cold" because, though not technically alive (or warm), it still lives forever.
Tease here meant in both the romantic sense (as if the maidens are teasing the male figures) and the literal sense, that is, to provoke or disturb. Thus neither the urn nor eternity can be understood by the speaker or, by extension, humanity.