du/dô pref. “should, so-as”
du is a verbal prefix that indicates the optative mood, a wish or hope.
However, if it undergoes a-fortification to produce dô, then it more likely indicates the subjunctive mood, or at least the optative mood with an imperative, decisive inflection, such as kadô (“and so”) in the first line of the Lament for Akallabêth, îdô (“now”), or tâidô (“once, then”).