Principal parts

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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. The concept originates in the humanist Latin schools, where students learned verbs by chanting them in the four key forms from which all other forms can be deduced, for example:

ferō – ferre – tulī – lātum ('to carry')

Not all languages have to be taught in this way. In French, for example, regular verbs can be deduced from a single form, the infinitive, and irregular verbs are too random to be systematized under fixed parts. But the concept can be carried over to many languages in which the verbs have some kind of "regular irregularity", i.e. irregularity always occurs at the same place in an otherwise regular system.

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.