With its first word, this hymn establishes the timeframe of its praise. "Now" both underlines the urgency of the speaker's call to praise (come sing now, medieval friends) and places the poem in the present. For Caedmon that meant c. 685 CE, but notice how this present is not fixed. "Now" is a relative marker of time, changing every second. For a reader today, "now" means the 21st century. In other words, Caedmon's Hymn is not tied to a particular era; it is eternally relevant.
the work of the Glory-Father, when he of wonders of every one, eternal Lord, the beginning established (3-4)
God himself may be eternal, but there are a lot of historical, dated events on his resume. Since he lives forever, time doesn't really apply to him, but he's still responsible for setting the clock ticking on everything else, including all the "wonders" of the universe.
he first created for men's sons heaven as a roof (6)
Like "now," "first" also establishes a timeframe, only now Caedmon is talking about an historical event: the creation of a livable universe for humans. It's also a relative marker of time, emphasizing that heaven was created before the earth.
then middle-earth mankind's Guardian, eternal Lord, afterwards made (7-8)
It's interesting to see the intersection of eternal with earthly time here. On the one hand, we have the relative, earthly time markers "then" and "afterwards," which date the historical creation of the earth. But in the middle we also have "eternal," an adjective applied to God, that reminds us that this dude has lived forever—no "first," "then," or "afterwards" for him.
eternal Lord (4, 7)
Since this title is the only one used twice, it must be important. Maybe "eternal" is the most important adjective to attach to God, since, if he doesn't last forever, all his other attributes—his holiness and might—will also disappear.