Question about the preposition "ab/ā"

Latin Learning Latin
1

I’m doing the exercises from Chapter 21 of Familia Romana (LLPSI).

In Exercitium 2, item #12:

Active: Fulgur caelum illūstrāvit =

My wrong answer (Passive): “Caelum ā fulgure illūstrātum est.”

Correct answer: “Caelum fulgure illūstrātum est.”

Why is the preposition “ab/ā” omitted here?

2

It’s not omitted – one could even say it’s not permitted. ab + abl. with a passive verb normally denotes human agents, while the bare ablative is used to denote instruments, the principal difference being that the former are conceived of as performing an action, while the latter are thought of merely as the force by means of which the event takes place. See Allen & Greenough § 405, in particular:

Note 2— The Ablative of the Agent (which requires ā or ab) must be carefully distinguished from the Ablative of Instrument, which has no preposition (§ 409).

occīsus gladiō
slain by a sword

BUT
occīsus ab hoste
slain by an enemy

Note 3— The Ablative of the Agent is commonest with nouns denoting persons, but it occurs also with names of things or qualities when these are conceived as performing an action and so are partly or wholly personified, as in the last example under the rule.

The “last example under the rule” is this:

nē virtūs ab audāciā vincerētur (Sest. 92)
that valor might not be overborne by audacity

(Audācia is, in a manner, personified here.)

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3

Thanks for the explanation and the link, they really helped!