Dative of Agent
When the passive periphrastic shows up — -ndus, -nda, -ndum + a form of sum — the person who must do the deed is in the dative, not the ablative. Carthāgō dēlenda est nōbīs says "Carthage must be destroyed by us," and the nōbīs is dative.
This is the trap: every other passive verb takes ā/ab + ablative for the agent, but the gerundive forces a switch.
Think of it as "the destroying-job for us" — Latin really is saying that the necessity belongs to the agent (cf. mihi est liber, "I have a book").
The same dative also turns up with perfect participles that have settled into adjectives (mihi dēlīberātum est) and, in poetry, with almost any passive verb (neque cernitur ūllī).
"X must be done by ME" — the dative names the person on whom the necessity rests.
With every OTHER passive verb the agent is ā/ab + ablative. The gerundive forces the dative — that is the whole rule.
See It In Action
— Sall. Cat. 52
Caesar (in Sallust's report) uses the textbook pattern: gerundive prōvidendum est + dative vōbīs. No ā vōbīs — the periphrastic forces the dative.
— B. C. i. 64
One nōbīs covers two gerundives (differendum, cōgitandum) — Latin is happy to share a single dative-of-agent across coordinated periphrastics.
— Plin. Ep. ix. 11
Impersonal periphrastic — there is no nominative subject, just prōvidendum est. The dative mihi still names the agent, and English most naturally goes active: "I must take care."
— Verg. Aen. i. 440
No gerundive here — Vergil uses the dative of agent (ūllī) with a plain passive (cernitur). This poetic extension (A&G § 375. a) is the second main place students meet the construction.
"X must be done by ME" — keeps Latin's grammar visible
mihi faciendum est = "it must be done by me"
"I must do X" — flips agent into subject, periphrastic into modal verb
mihi faciendum est = "I have to do it" / "I must do it"
"X must do/think/take care" — the active reading is the only readable one in English
nōbīs cōgitandum = "we must think" (literal: "it must be thought by us")
"I have decided / it is settled with me that…" — translate the dative as the subject of an English perfect
mihi dēlīberātum est = "I have decided"
Two ways to mark the doer of a passive verb. The trigger is purely grammatical: gerundive forces dative, every other passive takes ā/ab.
person on whom the necessity rests
nōbīs cōgitandum est
we must think (it must be thought-about by us)
person who did the action
ā Caesare laudātur
he is praised by Caesar
Tip: Look at the verb form FIRST. If you see -ndus/-nda/-ndum + sum, the agent will be a bare dative. If you see any other passive, expect ā/ab + ablative.
In Caesar's differendum est … nōbīs et de proeliō cōgitandum (B. C. i. 64), what is nōbīs?
Study Tips
- •When you see a -ndus/-nda/-ndum form + est, scan for a bare dative — mihi, tibi, nōbīs, vōbīs, sibi — that dative is the agent. No ā/ab is coming.
- •Translate the dative two ways and pick whichever sounds better: mihi faciendum est = "it must be done by me" OR "I must do it." The English passive is often clunky; the active reading is usually cleaner.
- •Watch for the rare ab override: when two datives would collide (a dative-of-agent next to an indirect-object dative), Cicero switches to ab + ablative just to clear the ambiguity (A&G § 374. a. N. 1).