When one of the left letters serves noon qalqalah letters saakinah or tanween, ikhfaa must be done. This is when the noon saakinah or tanween is hidden in the nose. Here, the mouth is to be shaped in a way such that it is “prepared” for the next letter.
For example, if the following letter is a taa with a dhammah then the mouth can say this by changing it into a small “o” shape, and the tongue places itself in the right place within the mouth. Benefits of speaking Arabic. This can occur across two words or be contained in one. Ikhfaa must be sounded for two counts, i.e., the time it takes to say the words, “one-two.”
You don't need to know all these idgham letters. It is however recommended that you learn the six ith-haar, and seven exceptions, letters and so when reading, do a quick check to see if the letter is not one of the six ith-haar letters or the exception letters above, then do ikhfaa. This takes a while to get accepted to, and soon you’ll forget you even have to do a check because you internalize it.
Besides, it’d be almost impossible to try and bring out an “nn” sound from your nasal passage when wanting to shape your mouth for a“h” sound for haa, or an “a” sound for alif.
Letters are now the place where the Hukum Tajwid words are connected without the Tanween. Some students [and I know quite a few] have trouble, in the beginning, trying to combine two words without the “nn” sound in between. They end up “leaning on” the laam or raa and saying them as though there is a shaddah on these two letters. However, there isn’t.
But here yourself may muqaddimah al jazariyyah complain that there is. Look at the example above. There is a shaddah on the laam in both events for the words likulli and lumazah. This shaddah is not there for shaddah purposes. It has been put there as a sign to the reader that the Tanween (or Noon Saakinah) is not to be pronounced at all, not even through the nasal passage.
The letters which are covered in rules three and four are the exception to this rule.
These letters are: laam raa yaa noon meem waa baa.
The following is rule two of four for the Noon Al-Saakinah/Tanween. We all recognize the children’s game, hide, and seek. Here, the Noon Al-Saakinah must be hidden [i.e., not pronounced clearly].
Rule Two: Ikhfaa — Al-sitr (where the Noon/Tanween is no pronounced, i.e., is hidden). The first rule placed six letters that follow the noon al-saakinah. In that case, it must be marked clearly of the remaining letters in the Arabic alphabet, the second rule, ikhfa letters applies.