I - Jussive* (commands)
P - Possibility
O - Optative (wishes)
D - Deliberative (questions)
* This works because the Latin J was spelled with an I. Also note that this covers the Jussive and the Hortatory. Essentially, these are "polite commands".
The Latin Lexicon (nicknamed Numen) is an online Latin dictionary (a dictionary of the Latin Language) and Latin grammar tool based on An Elementary Latin Dictionary (by Charlton T. Lewis). This online dictionary is different from any other you've ever used. It has been built from the ground up using AJAX technology to allow the fastest, most efficient and most useful user interface. Oh, and it includes macrons!