لماذا لا يجتمع ال التعريف مع التنوين

The definition does not match the grammatical marking: the tanween indicates indefiniteness while the addition specifies or defines, so they cannot be combined as their meanings contradict each other. Tanween signifies the end of a noun and the qualifier as being fully qualified, so combining them is not possible. Tanween is a sign of the end of the noun, and the qualifier is fully attached to it. The tanween does not mix with alif and lam for two reasons: the noun already carries alif and lam and cannot bear another addition, and alif and lam define the noun, making it heavier compared to the indefinite noun.