My personal preference is to put it as close to what it is modifying: only if..... Many others don't see this as important, because the meaning of the sentence is clear.
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AnonymousWhich one of the expressions is the correct one?
" I will be only able to help if "
" I will be able to help only if " << This one.
" I will be able to help if only "
You will also hear: I will only be able to help if ...
CJ