You may go out only if you come back soon. ✬

A is equivalent to B" has the same meaning as "A is true if and only if B is true. ✬

An integer is natural if and only if it is greater or equal to 0. ✬

The diagram above commutes if and only if f is an isomorphism. ✬

Tom can walk only if he has his cane. ✬

Tom told John that he would ask Mary to the dance only if John asked Alice. ✬

Tom will do that only if Mary tells him to. ✬

Tom will do that only if he has to. ✬

I'll do that only if Tom tells me to. ✬

I'll do that only if I have to. ✬