A sākin mīm is مْ, a mīm without a short vowel. In أَمْ مَنْ, the first word ends in sākin mīm; in وَمَا هُمْ بِمُؤْمِنِينَ, the mīm of هُمْ comes before bāʾ.
This one following consonant changes the rule. Do not decide from the written mīm alone. These combinations concern connected recitation; if you actually stop at the first word, the next-letter rule is not performed across that pause.
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Recognize mīm without a vowel
A sākin mīm is مْ, a mīm without a short vowel. In أَمْ مَنْ, the first word ends in sākin mīm; in وَمَا هُمْ بِمُؤْمِنِينَ, the mīm of هُمْ comes before bāʾ.
This one following consonant changes the rule. Do not decide from the written mīm alone. These combinations concern connected recitation; if you actually stop at the first word, the next-letter rule is not performed across that pause.