If any changes, what will you do?

(Grammar Questions I Couldn't Answer)

...actually I think I've got this one, but I'll throw it out to the blogosphere for a second opinion.

During an activity in which the students were supposed to brainstorm possible job interview questions, a student wrote the sentence "If any changes, what will you do?"

I corrected it as "If there are any changes, what will you do?"

The student was confused.  "But why do I need a full clause?" she asked.  "Don't we often use abbreviated phrases like 'If possible' or 'If necessary' without any subject in the If-Clause?"

Thinking on my feet, I said those were different because those were adverbial clauses.  Then I changed my mind and said they were adjectives.  (Both "possible" and "necessary" can modify nouns, right?  "a possible plan" or "a necessary plan")