Can someone explain agenda/agendum and data/datum?

[Fluffy24]

Should we strictly speaking be using agendum and datum more often than is customary?

I don't think I have ever seen agendum used. Agenda has become the widely accepted form. The oxford dictionary says they're interchangeable.

Yes, we should be using datum more often, though!

They're different in terms of grammatical origin, aren't they - datum is the past participle of Latin dare ('that which has been given') whereas agendum comes from the gerundive of Latin ago ('has to be done').

Not sure whether that makes a difference or not! But I'd be interested to see their history in English, if anyone has an OED subscription handy.

I thought the were plural/singular. Datum is rare, as you normally have more then 1 number.