Laudator Temporis Acti

Tom Shippey, The Road to Middle-Earth, rev. ed. (Boston: Houghton Mifflin, 2003), p. 328:

I would say that what hangs over the end of all Tolkien's fiction is not 'And so they all lived happily ever after', but the line from the Old English poem Déor, þæs ofereode, þisses swa mæg. This could be translated bluntly, 'That passed, this can too', but Tolkien translated it — see BLT 2, p. 323, for its importance to him and his writing — 'Time has passed since then, this too can pass'.

BLT = The Book of Lost Tales, where Christopher Tolkien wrote:

In the great Anglo-Saxon manuscript known as the Exeter Book there is a little poem of 42 lines to which the title of Déor is now given. It is an utterance of the minstrel Déor, who, as he tells, has lost his place and been supplanted in his lord's favour by another bard, named Heorrenda; in the body of the poem Déor draws examples from among the great misfortunes recounted in the heroic legends, and is comforted by them, concluding each allusion with the fixed refrain þæs ofereode; þisses swa mæg, which has been variously translated; my father held that it meant 'Time has passed since then, this too can pass'.

Kemp Malone, ed., Déor, 3rd ed. (London: Methuen & Co Ltd, 1961), pp. 23-24:

G. Shipley, Gen. Case p. 18, explains þæs and þisses as instrumental genitives, or, alternatively, genitives of measure, but it seems better to call them genitives of reference, or respect. Oferēode is used impersonally; see Shipley pp. 18 and 50. Ofergän is to be understood after mæg.

Morton W. Bloomfield, "The Form of Deor," PMLA 79.5 (December, 1964) 534-541 (at 535, where "lines" = the repeated refrain):

There have been possibly over a hundred different translations—as has been estimated—for these lines. It seems hard to believe that these five words of rather simple OE should give rise to so many variant translations, but it is nevertheless true.

Id. (at 536):

Literally then the words mean "in respect to that it passed away; in respect to this it likewise can or will (pass away).".... I render the whole refrain, polishing the literal translation, as "That passed away; so will ("shall" or "can") this."

Craig Williamson translated the refrain as "That passed over—so can this." Here are some lines from Déor without the refrain, translated by Williamson:

A man sits alone in the clutch of sorrow,
Separated from joy, thinking to himself
That his share of suffering is endless.
The man knows that all through middle-earth,
Wise God goes, handing out fortunes,
Giving grace to many—power, prosperity,
Wisdom, wealth—but to some a share of woe.

I haven't yet looked at:

• Theodor von Grienberger, "Deor," Anglia 45 (1921) 398-399
• Knud Schibsbye, "þæs ofereode, þisses swa mæg," English Studies 50 (1969) 380-381
• Murray F. Markland, "Deor: þæs ofereode, þisses swa mæg," American Notes and Queries 3 (November, 1972) 35-36
• Jon Erickson, "The Deor Genitives," Archivum Linguisticum n.s. 6 (1975) 77-84
• Gwang-Yoon Goh, "Genitive in Deor: Morphosyntax and Beyond," Review of English Studies 52 (2001) 485-499