Thats what I thought until a student gave me the following example;
The bus was empty except for me
In that sentence 'except' doesn't work (at least for me), but 'except for' does (or does it?).
Since I'm not a native speaker, I have to rely on what I've read or what I've been told somewhere. But this might work for you.
I think except for when it modifies a noun can be replaced by except, but except for when it doesn't modify a particular noun cannot.
(1) The room was entirely empty except for Morris. (adapted from COBUILD English Usage)
(2) Everyone was gone except for me. (Merriam-Webster Collegiate Dictionary)
In (1), except for Morris does not have any noun to modify within the sentence. Notice it cannot modify the room because the room except Morris doesn't make sense. It, rather, modifies the proposition, i.e., the room was entirely empty.
In (2), on the other hand, except for me does have a noun to modify, i.e., everyone, because everyone except me makes sense here. (Incidentally, note also that, right after the noun it modifies, except works, but not except for.)
I imagine you can use except in place of except for in (2), but not in (1). Your example is a case similar to (1).
Would you mind greatly if I quote your explanation to a student in another forum? Mike
No, not at all. Happy to be able to help. You can revise it any way you want, too. You can also combine it with Susie's explanation.
Would you mind if I asked you at which forum you are going to post it? Not that I want to watch what you do.:D
I just enjoy visiting such forums because I learn a lot from exchanges between students and teachers.