Only if is a conjunction used in conditional sentences to express a condition that must be fulfilled for the main clause to take place. Some synonyms for only if include provided that, on the condition that, as long as, subject to, and conditional on. These phrases all express the same conditional nature of the sentence and help to clarify the requirements necessary for the main clause to occur. Other options include in the event that, supposing that, and assuming that. All of these phrases point to the same idea of a necessary condition that must be met for something else to happen.