MORPHOLOGY: a process by which an inflected form of a word
is formed by changing the vowel of the base.
EXAMPLE: in English it is possible to derive the past tense of
strong verbs by substituting the base vowel by
another vowel: get:got, sing:sang and fall:fell.
Halle, M. & K.P. Mohanan (1985)
Scalise, S. (1984)
Spencer, A. (1991)