Indicates permission is granted only when a specific requirement is fulfilled. You may have dessert only if you eat your vegetables.
Pro Tip 2/3
Implicit Limits
Highlights a single situation where an action or event is allowed, excluding other possibilities. I'll lend you the book only if you return it next week.
Pro Tip 3/3
Emphasis on Condition
Accentuates the importance of the condition that must be met for the result to occur. He'll agree to the deal only if the terms are favorable.