What is the difference?

[takezo]

:D Hello! What is the difference between theses two english preps?
"If only..."
and
"Only if..."
I can't explain it to my esl friends. Thanks!
-Takezo

[gwendolinest]

• I would buy them if only they were cheap.

= They are not cheap; if they were, I would buy them; and I wish they were cheap!

• I will buy them only if they are cheap.

= If they are expensive, I won’t buy them.

()