principal parts of a verb
Definition - A traditional term for a list of the basic verb forms that are sufficient to show standard verb irregularities.

For example, in English this list consists of:
(1) the base or infinitive (look, see),
(2) the past tense (looked, saw), and
(3) the past participle (looked, seen).

Quotation -
Except for the verb be, we therefore need list only three forms to show irregularities in the verb: the base, the past, and the -ed participle. These three forms are known as the principal parts of the verb."
(Source: (Sidney Greenbaum, The Oxford English Grammar. Oxford Univ. Press, 1996))

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