Why does ‘lūdīs’ end in a short syllable in Ov. Ep. Sapph. 16?

Question

In Ovid’s Epistulae 16.152–153, the following two lines are found (‘eligiac couplet’, I believe is the term in English):

mṓre tuǽ gentī́s nitidā́ dum nū́da palǽstrā̆
   lū́dis et és nūdī́s fḗmina míxta virī́s.

(I used æ for the now more common ae in order to be able to place the diacritic for stress.)

For now, I have landed at this translation:

When, according to the custom of [your] countrymen, whilst in the wrestling ring, naked,
   in the games you were a woman mixing with naked men.

I take nūda to go with fēmina, nūdīs to go with virīs; I take nitidā to go with palǽstrā (thus -ā, not -ă), indicating the temporal aspect (and I believe dum should be taken as whilst). Due to the construction with dum, I chose the past imperfect for es rather than the present tense. Considering dum:

556. Dum (while) regularly takes the present indicative to denote continued action in past time. In translating, the English imperfect must generally be used.*

I cannot, though, for the life of me explain the short vowel in lūdis. I take it to be a poetic/historic accusative plural of lūdus, which should be lūdīs, though with my current analysis of the metre, that would be wrong. What is going on here? The interpretation of this might force a different translation, for which I would be most grateful.

Answer 1

It is a second-person singular verb form lūdis, “you play” (lūdō, lūdere).