Three weights are shown here for feminine forms ending in alif maqsurah. The changing vowels belong to the weight; the final written ى remains the identifying sign.
WeightExampleWhat to noticeفُعْلَىكُبْرَىdammah on the first radicalفَعْلَىمَرْضَىfathah on the first radicalفِعْلَىذِكْرَىkasrah on the first radical
The table compares the three vowel patterns; it does not claim that every word ending in ى must fit one of these examples.