'If only' and 'only if'

If only and only if are similar expressions that are used in different ways.

If only expresses a hope or wish:

If only I had a real choice in the matter.

She needed to get away from her job if only for a week.

If only the town had high-speed Internet access.

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Only if expresses a command or requirement:

He can stay on the team only if he completes his homework and improves his grades.

You'll lose weight only if you stick to your diet.

She was authorized to speak only if she remains anonymous.