Apparently you can't use "if" after a preposition. Do you have any opinions about if this is true?

The other thing, which I suppose I knew, is that "whether" can be fronted to be the subject but "if" can't.
It occurred to me that "whether" is more noun-like than "if" given both the above .
By the way, my missus says that I shouldn't ask for a "bit more wine" because it's "drop" for liquids. She tells me this a lot, as you can imagine, but does that make it true?