The letters of the Arabic alphabet are divided into 3 types on the basis of them being heavy or light.
1. Letters that are always Heavy: There are seven letters that are always heavy. They are compiled in the acronym (خص ضغط قظ). They are always heavy, regardless of their states (carrying a fatha, dammah, sukun or kasra).
2. Letters that are always Light: The rest of the letters with the exception of these three (ا - ل - ر) are always light.
3. Letters that are sometimes heavy and sometimes light:
a. The Letter ا - This letter follows the condition of what comes before it. If the letter preceding it is a heavy letter, then the ا will be heavy, and if the letter preceding it is a light letter, then the ا will be light.
Examples are as follows:
كان (this ا will be light because ك is a light letter)
قال (This ا will be heavy because ق is a heavy letter)
مرضا (this ا will be heavy, because ض is a heavy letter)
b. The Letter ل - This letter is always light except when in the name of Allah الله , it is preceded by a dammah or a fatha. In this case it is heavy.
Examples are عبدُ الله - رسولَ الله these are heavy examples. As for when it is light, an Example is بسمِ الله and so on.
c. The Letter ر - This letter has many rules but can be summarized into the following:
[1] ر is heavy when it has a dammah or a fatha, or if it has a sukun and the letter before it has a dammah or a fatha
[2] ر is light when it has a kasra, or if it has a sukun and the letter before it has a kasra or if the letter before it is a ي.
The Five Levels of Heaviness
1. The strongest level is when a heavy letter has a fatha and is followed by an ا. Example خَالدين
2. The next strongest level is when a heavy letter carries a fatha only or if it has a sukun and it is preceded by a letter with Fatha. Example خَبير
3. The middle level of heaviness is when the heavy letter has a dammah or if it has a sukun and it is preceded by a letter with a dammah. Example متقُون
4. The next level is when a heavy letter has a sukun and it is preceded by a Kasra. Example is مِقدار
5. The least level of heaviness is when a heavy letter has a kasra. Example بصِير