With regard to the root letters (الحروف الأصلية) of a verb, they are of two types:
Each of these two types is further divided into two categories:
(1) (مجرد) - the (الماضي) consists of root letters only, without any extra letters.
(2) (مزيد فيه) - the (الماضي) has root letters plus extra letters.
This results in the following four categories:
http://al-xr.ifrance.com/ALFI3L.PHP
Rujukan:
Maulana Ebrahim Muhammad. 2006. FROM THE TREASURES OF ARABIC MARPHOLOGY. Camperdown South Africa: Madrasah In'amiyyah.
