Verbs are divided into two categories: strong verbs (الصَّحيحُ ), which are also called sound verbs, and weak verbs (الْمُعْتَلُّ).
A sound or strong verb is a verb that has no long vowels ( ي ا و) in its root, such as كَتَبَ (to write) and دَحْرَجَ (to roll).
A weak verb is a verb that has a defective letter in its root, such as وَعَدَ (to promise) and رَمى (to throw). The defective letters are (ا و ي ).
The weak verbs are: assimilated, hollow, and defective verbs.
Assimilated verbs are verbs whose first root letter is و or ي.
Example: وَصَلَ (to arrive).
Hollow verbs have a weak letter as the second letter of the root.
Example: قالَ (to tell).
Defective verbs have a weak letter as the last letter of the root.
Example: مَشى (to walk)
These weak letters are often deleted, or changed in some way, depending on the phonetic environment.
See also: