VERBUM supernum prodiens, Supernal Word proceeding,
nec Patris linquens dexteram, yet not leaving the Father’s right hand,
ad opus suum exiens, departing for His work
venit ad vitae vesperam. He came at life’s evening.
Here is the original:
Verbum supernum prodiens, Supernal Word proceeding,
a Patre olim exiens, once going forth from the Father,
qui natus orbi subvenis who was born to come to the world’s aid
cursu declivi temporis… when time in its course sloped toward dusk…
As you can see, the first line is repeated wholesale, along with the idea of Christ’s “departing” (exiens) from the Father and coming to earth in time’s evening (venit ad vitae vesperam = cursu declivi temporis).
Yet I think A.S. Walpole in his comments on the original Latin hymn may be right that Thomas misunderstood the force of the first line in the original. In the original hymn, the first stanza is divided symmetrically between two ideas: Eternal Generation (lines 1-2; n.b. olim) and Incarnation (lines 3-4). But Thomas in his first two lines contrasts prodiens with non linquens in his own first two lines, thereby seeming to make line 1 about the Incarnation (along with lines 3 and 4) and line 2 about the fact that He was nevertheless at the same time still present with the Father.
It is possible, of course, that he did not misread it, but creatively reappropriated it–the line was to hand and could be reused in a new way. Even if he did misunderstand, a mitigation immediately presents itself: if Homer can be permitted to nod, so can Thomas!