There are two rules related to Meem Saakin. Meem saakin means that Meem which has a sukoon/jazm on it.
1. Ikhfa. Whenever there is a meem saakin followed by a Baa, ikhfa will be done. The sound of the meem will not be completely pronounced, and the sound will be lengthened. Example:
وَمَا هُم بِمُؤْمِنِينَ
2. Izhar. If there is ANY other letter other than Meem, the meem will be pronounced completely, and the sound will NOT be lengthened. Example:
وَلَهُمْ عَذَابٌ عظِيمٌ
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