G. H. Armstrong, English Grammar and Composition for Public Schools
“ A verb that forms its past tense by changing the vowel of the present, and its perfect participle by adding n or en to the present, is a verb of the strong or old conjugation; as, fall, fell, fallen.Note.—Verbs of the old conjugation are called strong because they form their past tense within themselves. Verbs of the new conjugation are called weak because they form their past tense by the aid of an additional syllable. ”
