Ever wondered why while reading the Qur’an do we assimilate some letters and not others?
A shaddah (w on top of the letter) is used when the first consonant has a sukoon (no vowel) on it and the second one is followed by a vowel. For example when you have a word that starts with ال followed by one of these letters u make shaddah 
on top of the word: ت ث د ذ ر ز س ش ص ض ط ظ ل ن
on top of the word: ت ث د ذ ر ز س ش ص ض ط ظ ل ن These are called sun letters
So u will say الشَّمس , because of the ش
Therefore instead of Al shamsu it will be pronounced Ash-shamsu.
The rest of the letters are called moon letters. In thee letters you do not place a shaddah on top and the words are not assimilated.
For example: القَمَر will be pronounced al- Qamar and not Aq-Qamar.
The fourteen moon letters are: ء ب ج ح خ ع غ ف ق ك م و ي ه
THE MUSLIM MONK 