It's a special case. There are special cases in which the "alef" is omitted , one of those special cases is the words for "man = امرؤ" and "woman = امرأة" when defined with "الـ" , so they become "المرء" and "المرأة", it's not grammatical, it's for ease of pronunciation, the "Alef" in the indifinite version of the word is actually an addition for the same reason. To make it simpler: - Originally, the word is "مرأة" "mar-'ah" . - For pronunciation reasons, Arabs usually added the "alef" so it became "امرأة" "em-ra'ah". - When the word is defined with "الـ" , it reverts to the origin, so it becomes "المرأة" "al mar-'ah".

First of all - women - is written like this - امرأة - امرأة is singular woman مرأة plural women So when use المرأة you mean women not woman

The alef ا in إمرأة stand for "a" woman. So you have to remove the "ا" so that when you say "the woman" it's not "A the woman."

على ما يبدو من سؤالها أنه لماذا لا تكون "الامرأة" بدلا من "المرأة" بعد إضافة "ال" لها. وعن سبب حذف الألف من امرأة . هذه الألف " اِمْرأة" همزة وصل والكلمة مبدوأة بساكن "مْرأة" والعرب لا يبدؤون بساكن فحركت الكلمة بالألف لتصبح " اِمْرأة" وأظن أنه يجوز حذفها أو بقاؤها. . مثل "ابن" و "بن" وأهل الاختصاص أعلم مني ، والله أعلم

شكرا Heba