Agendum can be a noun with the meaning of doing. It cannot be pluralised. However agenda is a gerundive with the meaning of things which ought to be done. Subtle difference.
You've got to make allowances for me, Jackdaw. Not the best starts in life. Not much money. Parents ran away from home three times. You know, all that Les Dawson stuff.
You're trying to make me weep here.
Basic definition is that a gerund is a verbal noun whilst a gerundive is a verbal adjective implying necessity. Cato the Elder would always finish his speeches in the Senate with the words, "Delenda est Carthago". (Carthage must be destroyed)