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.