In the English language, verbs are divided into two groups: the strong and the weak. Weak verbs are those which simply add the letters ED onto the basic verb in order to form the other principal parts; an example is the verb WALK, WALKED, WALKED. A strong verb is one which changes its spelling in order to form the three principal parts; an example is the verb SING, SANG, SUNG.
Greenblatt, M. H.
"The Strong Vs. The Weak,"
Word Ways: Vol. 5
, Article 12.
Available at: http://digitalcommons.butler.edu/wordways/vol5/iss2/12