In A new latin composition by Charles E. Bennett one may find following statement to translate into latin (from Lesson IV):
This circumstance made the troops more courageous.
My attempt at translation is:
Res haec milites fortioribus egit.
I used fortior in ablative, however I don't know whether it is correct.
This leads to the more general question: what should be the case of the adjective Z in "X made Y (more) Z" construction?
I would use facere and accusative instead of agere and ablative:
Res haec milites fortiores fecit.
However, a literal translation of "to make something something" is not the only option. I would phrase the cause as a causal ablative. (Note that it is the cause, not the outcome, that is in ablative in this approach.) For circumstances I suggest rerum condicio, but there are other options. My offer for a translation is:
This circumstance made the troops more courageous.
Milites rerum condicione fortiores erant.
(lit. "The soldiers were more courageous due to the circumstances.")
Many choices are a matter of style. The most important correction to your attempt is a grammatical one: fortiores should not be in ablative. If you make something something directly, both somethings are in accusative. In passive they are both in nominative (milites fortiores facti sunt).
