«إِنْ أَخْبَرْتَنِي بِقُدُومِ فُلَانٍ»
“If you inform me of his arrival.”
«إِنْ أَخْبَرْتَنِي أَنَّ فُلَانًا قَدِمَ»
“If you tell me that he has arrived.”
In the first, ba of attachment binds the statement to the actual event reported. In the second, the condition may focus on performing the act of stating a particular proposition, even if the proposition is untrue.
The difference between a truthful report about an event and the act of saying something matters for determining whether a conditional legal effect occurs.