Idgham (assimilation) means to enter one thing into another. When two letter having a similar point of articulation or similar kind appear side by side first letter with ْ- (saakin ) and the second letter with Tashdid ّ- with following characteristic
(1)Same letters Example د and د ; ت and ت (Idghaam Mithlayn)
(2) Letters with similar origin Example ق vs. ك or م vs. ب or ل vs. ر or ن vs. ر Idghaam Mutaqaaribany
(3) Related letters Example ت vs. ط or ط vs. ت or ت vs. د (Idghaam Mutajaanisayn)
In such cases the letter with ْ- (saakin) will assimilate (suppressed or not pronounced) with the second letter.
There are many exceptions to this rule namely, such as nun saakin and letters having Tashdid - Idgham (assimilation);Iqlab (conversion); Izhar (sharp pronunciation) and Ikhfa (suppressed pronunciation) Some of the basic rules will be explained in the following lessons.