The agent in Latin is typically expressed by ab + the ablative case: haec a te facta sunt = "these things were done by you." With the passive periphrastic, however, the Romans used the dative case to indicate the person who ought to do the necessary or obligatory thing. This looks like an agent and can be easily translated by the same formula, but strictly speaking, the dative just indicates the person for whom an obligation or duty exists. You may remember that "for" is the basic notion of the Dative Case. Now, if you think of the English word agenda, you have a perfect model for understanding the Latin gerundive with the dative of agent. Suppose I gave you a list of things to do. I could say "this agenda is for you" or I could say "these things are to be done by you". Both sentences in Latin would be haec agenda tibi sunt = "for you these are the things-to-be-done."