«إِنْ صُمْتَ الشَّهْرَ فَأَنْتَ كَذَا»
“If you fast the month, this consequence follows.”
«إِنْ صُمْتَ فِي الشَّهْرِ فَأَنْتَ كَذَا»
“If you fast during the month, this consequence follows.”
In the first, the month itself is the duration whose fasting is stipulated. In the second, fi specifies the temporal setting in which an act of fasting occurs, so the relevant performance within the month may suffice.
The conditions differ: fasting the entire month is not the same as fasting at some time in the month. A single particle changes the scope of the condition.