In language learning, the principal parts of a verb are the series of key forms which the student has to learn by heart in order to be able to conjugate the verb through all its forms.

In English, the verb love forms all its parts systematically (loves, loved, loving), and as these can all be deduced from the basic form, no principal parts have to be learned. With the verb sing, on the other hand, the forms sang and sung cannot be deduced, so the foreign learner of English must memorise sing-sang-sung. From these, all other forms (like sings or singing) can be deduced.

In Latin, most verbs have four principal parts. The verb "to carry" has the parts fero-ferre-tuli-latum. Fero is the 1st person singular form of the present ("I carry"), but the infinitive ferre ("to carry") is also needed to deduce all the forms of the present stem. The perfect stem tuli ("I carried") and the supine stem latum (required for the perfect passive) are quite irregular in this case, but even with many verbs which are basically regular, they have to be learned in the same way.