A Noon with a vowel sign is pronounced with the corresponding vowel sign. As for a Noon with a sukun (or without a vowel sign) it is written in the Mushaf slightly differently, based on one of the four rules that apply to its pronunciation.
1. Clear Enunciation الإظهار - This ن is represented in the Mushaf as a ن with the top part of a ح.
This rule is applied when the ن is followed by one of the six throat letters (ء ه ع ح غ خ).
Its ruling is to pronounce the ن clearly without its natural nasal sound, nor merging or hiding it in the following letter.
Examples are: من ءامن - ينهون - من عمل - المنخنقة - من غير - ينحتون
2. Changing/Converting الإقلاب - This is when the ن is followed by a ب only. It is represented in a Mushaf by a ن with a م on top of it.
This change or conversion happens in three steps:
a. The ن is changed into a م
b. The nasal sound of the م is held for two counts
c. The م itself is hidden in the ب
Examples are: من بعد - أنبياء
3. Merging الإدغام لإقلاب - This ن is represented in the Mushaf without anything on top of it. This rule applies when a ن with sukun is followed by one of these six letters (يرملون).
This rule is further divided into two types:
a. Complete Merging إدغام كامل - This is when the letters ر and ل come after a ن with sukun or a tanween. The ن will be completely merged into the following letter without any enunciation of the ن or its characteristic of ghunnah (nasal sound).
Examples are: من ربهم - ولكن لا
b. Partial Merging إدغام ناقص - This is when the remaining four letters (ي ن م و) come after a ن with sukun or tanween. The ن will be partially merged into the following letter, not using the point of articulation of the ن but retaining the ghunnah of it. The ghunnah will be held for two counts.
Examples are: من يعمل - من نعمة - من مال - من ولى
Exception to the Rule - **Merging only occurs when the ن with sukun or tanween is at the end of one word, and the following letter that will cause the merging is in another word immediately following it. If the ن with sukun and the following letter of merging are in the same word, the ruling for the ن would be “Clear Enunciation - الإظهار”. There are only four of such words in the Mushaf. They are: دنيا - صنوان - قنوان - بنيان
4. Hiding الإخفاء - This ن is also represented with nothing on it in the Mushaf.
This rule applies when all the other letters of the Arabic alphabet (asides from the letters mentioned in the previous three rules (ء ه ع ح غ خ - ب - ي ر م ل و ن)) come after a ن with sukun or a tanween. This rule is hiding, and it entails retaining the ghunnah of the ن letter along with partially withdrawing the tongue from touching the point of articulation of the ن completely.
Examples are: أنداد - تنزيل - من شر