Change of tense from present to imperfect

Question

In the Metamorphoses there is a line as follows:

Nil prosunt artes erat inmedicabile vulnus

Why would the first sentence be in the present tense, but the second one be in the imperfect? I would have expected est instead of erat.

Answer 1

It's actually quite common to switch from the historic present to other past tenses. Here are some examples from Pinkster's Oxford Latin Syntax, vol. I, 7.16, with the historic present in bold and narrative past tenses in italic:

Interea ea legione, quam secum habebat, militibus que, qui ex provincia convenerant . . . fossam . . . perducit. (Caesar, De Bello Gallico, 1.8)

(Note the use of the pluperfect to indicate a completed action in the past, despite the narrative present in the main clause.)

Sermonem ibi nobiscum copulat. ait sé peregrinum esse huius ignarum oppidi; . . . nos hominem ad te adduximus. (Plautus, Poenulus, 655-58)

(This example is especially relevant because, after a long narration in the present tense, the "switch" to the end of the story is signaled by the perfect tense.)

sic ait et dextra crinem secat, omnis et una
dilapsus calor atque in uentos uita recessit. (Vergil, Aeneid, 4.704-705)

(Again relevant because of the shift to the perfect tense at the end.)

My own observation about your passage from Ovid is that it employs the present tense to create a vivid impression of immediacy. Present and past tenses are used throughout the narration, but this particular sequence contains a high concentration of present tenses, and the final imperfect (at least to my ear) allows us to "zoom out" and again see the event as happening in the past:

expalluit aeque
quam puer ipse deus conlapsosque excipit artus,
et modo te refovet, modo tristia vulnera siccat,
nunc animam admotis fugientem sustinet herbis.
nil prosunt artes: erat inmedicabile vulnus. (Ovid, Metamorphoses 10.185-89)

English might employ a similar device: "He tries the door. He knocks on the door. He cries for help. All was lost."

As you can see, there's a pretty free change from past tenses to the historic present, and Pinkster's discussion offers many more such examples. Pinkster (pg. 404) notes:

Ex. (a) [i.e. the Plautus example above] shows that, in order for a hearer/reader to be able to interpret a present tense as 'historic', there must be a sufficient number of 'past signals' in the context or in the communicative situation. Narratives rarely start or end with (a) historic present(s). (emphasis mine)

He goes on (of course) to note exceptions.