''if only" or "only if"?

[alysa]

I came upon a sentence in my reading like this: we knew that we had marvelous powers within our grasp, if only Caraaierra could be kept at his work.Here,the writer uses "if only",but is it can be changed into"only if"? I'm wodering it. Thank you.

[Casiopea]

They're not interchangeable.

only if <"only" modifies the if-clause>
if only <"only" doesn't modify the if-clause>

[river]

"If only" expresses a wish. "If only you would listen."
"Only if" means never except when. "I'll listen only if you say what I want to hear."