Grammar: Dum + subjunctive
Like the temporal conjunction cum, there are some other temporal conjunctions -- antequam, priusquam, dum -- which are used sometimes with indicative verbs and with subjunctive verbs.
Indicative. Indicative verbs are used to describe factual situations in time:
» antequam pecuniam perdidit, Marcia valde felix erat. Before she lost her money, Marcia was very happy.
» dum pecuniam habet, Marcia felix est. So long as she has money, Marcia is happy.
» dum Samson dormit, tonsor eius caput rasit. While Samson slept, the barber shaved his head.
Subjunctive. Subjunctive verbs are used to express purpose or expectation (or failed expectation):
» priusquam eius litteras legeret, Marcus mortuus est. Marcus died before he could read the letter.
» exspectamus dum Marcia veniat. We are waiting until Marcia comes.