Compare the two endings directly: both can mark feminine forms, but their written shapes are different.
Maqsurah ends in written ى: كُبْرَى. The preceding letter has fathah.
Mamdudah ends in اء: حَمْرَاءُ. An alif is followed by hamzah.
Both can mark the feminine, but their written endings and listed weights are different.
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Alif al-Ta'nith: Maqsurah and Mamdudah
Compare the two endings directly: both can mark feminine forms, but their written shapes are different.
Maqsurah ends in written ى: كُبْرَى. The preceding letter has fathah.
Mamdudah ends in اء: حَمْرَاءُ. An alif is followed by hamzah.
Both can mark the feminine, but their written endings and listed weights are different.