الفعل الذي قصد اقترانه بالشرط فله مع «إن» مقام ليس له مع «إذا»
A verb intended in a conditional construction has a station with “in” that it does not have with “idha”.
Both particles occur in conditions, but they can imply different orientations. In broad traditional usage, in can foreground contingency or uncertainty, while idha often suits a condition viewed as expected or likely to occur. Actual interpretation remains dependent on context and the construction.
The contrast shows that replacing one legitimate Arabic particle with another can change the rhetorical station and intended force. This is an introductory application; the extended treatment follows in the work.