Principal parts

In language learning, the principal parts of a verb are those forms that a student must memorize in order to be able to conjugate the verb through all its forms. Although the term 'principal part' is usually applied to verbs, the same phenomenon can be found in some languages in nouns and other word types. It is normally restricted to regular verbs, nouns, etc., and applies to languages where the regular paradigm is based on more than one underlying form. It does not cover random irregularities.

The principal parts of an English verb are the infinitive, preterite and past participle. Lists or recitations of principal parts in English often omit the third principal part's auxiliary verb, rendering it identical to its grammatically distinct participial form. For example, the verb "to take" has the principal parts take–took–(have) taken. The verb "to do" has do–did–(have) done and the verb "to say" has say–said–(have) said.

Most verbs are regular enough not to require a knowledge of the principal parts. For example, the verb love derives all its forms systematically (love, loves, loved, loving), and since these can all be deduced from the basic form (the citation, dictionary, or lexicographic form, which in English is the bare infinitive), no other principal parts have to be learned. With irregular verbs like the verb sing, on the other hand, the forms sang and (have) sung cannot be deduced, so the learner of English must memorize three principal parts, sing–sang–(have) sung. From these, all other forms (like sings or singing) can be deduced.

There are a handful of verbs in English that are so irregular that the principal parts are not enough to conjugate them fully. For example, the verb "to be" has the principal parts be–was/were–(have) been, showing an irregular past tense (was for the first- and third-person singular, and were for the rest), and an entirely irregular present tense (using am, is and are instead of forms derived from the first principal part be).

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