The simple conditional is one of a group of connectives whose other members can be expressed using it and negation.
5.2.1. Only if
If the simple conditional trims the content of an unconditional assertion, a second conditional offers a trimmed denial, ruling out the truth of the main clause in cases where the subordinate clause is false.
5.2.2. Necessary and sufficient conditions
The implicatures of an only-if conditional are associated with the idea of necessary conditions while the implicatures of an if conditional are associated with the idea of sufficient conditions.
5.2.3. Unless
Although it may be obscured by implicatures, the content of unless is provided by the phrase if not, a common dictionary definition for it.
5.2.4. Three forms compared
The implicatures associated with conditionals can make it difficult to distinguish the three conditionals but, once they are distinguished, some couple mnemonic devices point to their symbolic forms.