A student was given this gap-fill question:
"We got stuck in traffic;______, we would have arrived on time."
The test maker had hoped the students would use "otherwise" or something like that.
One student wrote "if not."

"We got stuck in traffic; if not, we would have arrived on time."

It doesn't look right to me. I suppose it's an elision of 'if we had not,' yet it seems kind of murky and wrong somehow.
Is it, in fact, correct? If not, can somebody help me explain the problem to the student?
It just doesn't sound correct to me! "Otherwise" would have been the correct answer if I had written the test question.

I think the term you want is ellipsis.

I wouldn't say it's wrong, although I agree that it seems slightly murky.

Was it OK to use two words in the test?

Students: We have free audio pronunciation exercises.
Quite right, ellipsis is the word. Thanks.
Site Hint: Check out our list of pronunciation videos.