The letters of Idgham (naqqis and tam) are ن و ل م ر ي and they are divided in two distinct group (1) Idgham-e-Naqqis (partial assimilation) (2) Idgham-e-Taam (complete assimilation)
Idgham-e-Naqqis (partial assimilation of letters)
When nun saakin نْ appears at the end of a word (including Tanween ً ٍ ٌ) and the first letter in the next word is either ي و ن having Tashdid ّ In such cases there will be a slight alteration in pronunciation - Nun saakin نْ become partially assimilated (blended) and the connecting letters ي و ن will be pronounced with a partially nasal sound (Ghunnah)- If the letter following Nun saakin نْ is letter Meem م then the letter Nun نْ will not be pronounced - It will read with meem (mashaddad Ghunnah) The symbol of Idgham is Tashdid ّ that appears on the letter of Idgham.
Note: An Idgham or assimilation is not possible within a single word

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