In language learning, the principal parts of a verb are the most fundamental forms of a verb that can be conjugated into any form of the verb. 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.
Classical languages
Latin
In Latin, most verbs have four principal parts. For example, the verb for "to carry" is given as portō – portāre – portāvī – portātum, where portō is the first-person singular present active indicative ("I carry"), portāre is the present active infinitive ("to carry"), portāvī is the first-person singular perfect active indicative ("I carried"), and portātum is the neuter supine. Most of the verb forms in Latin derive from the first two principal parts: portābō, "I shall carry", is derived from the root portā-, taken from the present infinitive. However, all active perfect forms are derived from the third principal part (so portāveram, "I had carried", is taken from portāv-) while the perfect participle (portātus, portāta, portātum, "having been carried") is derived from the supine and is used to form the perfect passive participle with the auxiliary verb sum (such as portātum est, "it has been carried"). The auxiliary verb is often dropped when writing poetry in Latin. For many Latin verbs, the principal parts are predictable: portō shown above uses a single stem, port-, and all principal parts are derived from them with the endings -ō – -āre – -āvī – -ātum. Others have more complicated forms: regō ("I rule") has the perfect form rēxī and perfect participle rēctum, derived as *reg-sī and *reg-tum. A handful of verbs, such as sum – esse – fuī – futūrum ("to be"), are simply irregular. A number of verbs have fewer than four principal parts: deponent verbs, such as hortŏr – hortāri – hortātus sum, "to exhort", lack a perfect form, as do semi-deponent verbs, such as audeō – audēre – ausus sum, "to dare"; in both cases, passive forms are treated as active, so all perfect forms are covered by the perfect participle. A handful of verbs are also defective, including the verb ōdī – ōdisse, "to hate", which only has perfect forms derived from a single stem.
Ancient Greek
Verbs in Ancient Greek have six principal parts: present (I), future (II), aorist (III), perfect (IV), perfect middle (V) and aorist passive (VI), each listed in its first-person singular form:
Part I forms the entire present system, as well as the imperfect. Part II forms the future tense in the active and middle voices. Part III forms the aorist in the active and middle voices. Part IV forms the perfect and pluperfect in the active voice, and the (exceedingly rare) future perfect, active. Part V forms the perfect and pluperfect in the middle voice, and the (rare) future perfect, middle. Part VI forms the aorist and future in the passive voice. One principal part can sometimes be predicted from another, but not with any certainty. For some classes of verbs, however, all principal parts can be predicted given the first one.
Sanskrit
Rather than providing the lexical principal parts like in Latin and Ancient Greek dictionaries, dictionaries in Sanskrit studies provide a single verbal root, which systematically derives every form by applying a set of rules. Any "irregularity" is expressed at this root through ten classes (or gaṇá) of present-stem formation.
Germanic languages
English Excluding four common irregular verbs, the principal parts of all other English verbs are the infinitive, preterite and past participle. All forms of these English verbs can be derived from the three principal parts. Four verbs have an unpredictable 3rd person singular form and the verb "to be" is so irregular it has seven separate forms. 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 bet" has bet – bet – (have) bet and the verb "to break" has break – broke – (have) broken. With irregular verbs the simple present 3S (he, she, it) is derived from infinitive+'s' with the exception of spelling changes such as catch – catches, fly – flies and teach – teaches, which follow the same rules for regular 3S verbs.
Examples of irregular verbs and their principal parts
Regular verbs Most verbs are regular enough that all forms can be derived directly from the infinitive. For example, the verb love derives all its forms systematically (love, loved, loving, has loved, loves), 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. The present 3S (he/she/it) is derived from the infinitive+'s' with the exception of verbs ending in a single -o, or ending in -s, -x, -z or the digraphs -sh, -ch, in which case it is derived from 'infinitive+es'. With three irregular verbs (and their derivatives) the 3S has to be learnt independently (e.g. has, does, undoes, redoes).
Highly irregular verbs There are three verbs (in addition to their derivatives) with an irregular third person singular form in the present tense. As a result, three principal parts are insufficient to conjugate these fully.
To be The verb "to be" is completely irregular having seven separate forms.
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