What are the harsh and soft letters in the Arabic alphabet?
Answer
There are different characteristics for the letters in the Arabic alphabet. We assume you are referring to
, which means strong, and 
, which means softness, and a third characteristic between these two, which is 
, meaning moderate. You can read about these characteristics and their letters in the following link:
http://www.abouttajweed.com/the-characteristics-of-the-arabic-letters/the-characteristics-of-the-letters-part-3-ash-shiddah-at-tawassit-ar-rakh.html .