Ablative of Agent
When a passive verb names the person who did the action, Latin marks that person with ā (or ab before a vowel) plus the ablative — laudātur ab hīs, "he is praised by these."
The trap is the bare ablative right next door: occīsus gladiō means "slain by a sword" (the instrument), but occīsus ab hoste means "slain by an enemy" (the agent). Same English "by," two different Latin patterns.
Rule of thumb: a person or personified force takes ā/ab; a thing or tool takes the bare ablative. Animals can go either way depending on whether the author treats them as actors or as means.
Both translate as English "by" — Latin splits them on whether the doer is a person or a thing.
Use ā before a consonant, ab before a vowel or h. The verb must be passive (or feel passive).
See It In Action
— Hor. S. i. 2. 11
Two passive verbs, two ab phrases — Horace uses the textbook pattern. The agents (hīs, illīs) are people, so ab is required.
— B. G. i. 21
Compare Caesar's other phrasing per explōrātōrēs Caesar certior factus est (BG i. 12): same scouts, but per + acc. treats them as the channel rather than the willing source.
— B. G. vii. 47
Indirect statement carries the agent construction in: a Romanis attaches to the passive infinitive teneri, naming who is doing the holding.
— Cic. Cat. ii. 12
Cicero defends his decision in oratio obliqua. a me with the perfect passive infinitive eiectum esse names him as the agent — exactly the personal-source role ab marks.
Both English-translate as "by." The Latin difference is whether the doer is a person (or personified force) or a tool.
person who did it (with passive verb)
occīsus ab hoste
slain by an enemy
tool or instrument (no preposition)
occīsus gladiō
slain by a sword
Tip: Ask: is the "by" word a person? If yes, it needs ā/ab. If it is a thing or tool, the ablative stands alone.
In Vergil's vulnus … inflictum ab Achille, what role does ab Achille play?
Study Tips
- •When you see ā/ab + ablative, look for a passive verb nearby — the prepositional phrase is almost always telling you who did the action.
- •Drill the contrast pair occīsus gladiō (means) vs. occīsus ab hoste (agent) until the test is automatic: person → ab, thing → no preposition.
- •Watch for per + accusative (per explōrātōrēs) — Caesar uses it when scouts are the channel of information rather than the willing source. It is agent-adjacent but technically means.