In general, you can’t re-arrange idiomatic expressions too much without losing the idiom. Thus,
1. The shit hit the fan
2. The fan was hit by shit
Reading (2), most of us probably envision literal shit literally hitting a literal fan before we re-compute the sentence as a passivized idiom. Still, it’s a legitimate and grammatical sentence. I can imagine someone using it for extra comedic effect.
However, consider the following idiomatic expression and its passive equivalent:
3. It rained cats and dogs (or, It was raining cats and dogs)
4. #Cats and dogs were raining
5. *Cats and dogs were raining by it
Unlike (2), sentences (4) and (5) are simply ungrammatical; actually, (4) is grammatical but semantically unacceptable because its theta grid is all mucked up.
What gives? Why can’t we passivize the idiomatic expression “It rained cats and dogs” the way we passivized “The shit hit the fan”? I haven’t seen this problem addressed in linguistics journals, but I’d bet a bottle of nice Scotch that it has to do with the expletive.
In a generative framework, passive sentences are structurally equivalent to their active counterparts. “The boy hit the ball” becomes “The ball was hit by the boy” via a simple transformation triggered by the addition of auxiliary “was.” Passivizing “hit” with the auxiliary blocks the verb’s ability to assign case to the DP “the ball.” So, to get case, the DP moves to spec-TP, and “the boy,” which was the subject in the active sentence, moves to an optional adjunct phrase (typically a by-phrase).
These movements cannot occur with the idiomatic expression in (3) because the active-voiced idiom contains an expletive in spec-TP. Passivizing the verb into “were raining” would mean moving the expletive to an optional adjunct phrase, as in (5). But expletives can’t appear in prepositional phrases! They aren’t allowed to surface in X-governed nodes. That explains why (5) is ungrammatical and why (4) sounds meaningless.
Now, while I was working on this fun linguistics puzzle, someone brought to my attention that it’s possible to passivize the following sentence, which is not idiomatic but does contain an expletive:
6. We found it to be raining.
7. It was found to be raining.
However, the expletive in this passive sentence generates in relation to “found”, an Exceptional Case Marking verb, and an infinitival to-be clause. So, the active version of this sentence can be analyzed simply as a small-clause structure:
[We found [it to be raining]]
The passive sentence, (7), generates by moving the expletive from spec-TP in (6) to spec-TP in (7) so that it can receive case. In other words, expletive it never needs to surface in an X-governed node like it would in (5). So, the sentence can be passivized.