if only—
1. is used to express a strong desire or hope that was not fulfilled:
- If only I knew at the time that she would be famous one day!
2. even though (is used to introduce the clause of concession):
- This may mean going on a trip, if only for a day or two, or an extended vacation of several weeks.
only if—not at all unless:
- A Commonwealth citizen or other foreign nationals can apply but only if they are resident in the UK.