Original sentence: For example, "I'll buy you an ice cream if you pass the exam" is meant as a biconditional, since the speaker doesn't intend a valid outcome to be buying the ice cream whether or not you pass the exam (as in a conditional).Slightly more formal, one would say "b implies a and a implies b".The only unambiguous way of stating a biconditional in plain English is of the form "b if a  and a if b"
Edited sentence: Another is "a if and only if b".For example, "I'll buy you an ice cream if you pass the exam" may be meant as a biconditional, since the speaker doesn't intend a valid outcome to be buying the ice cream whether or not you pass the exam (as in a conditional).One unambiguous way of stating a biconditional in plain English is of the form "b if a  and a if b".Slightly more formally, one could say "b implies a and a implies b"