Principal parts (verb)
The principal parts of an irregular verb[1] are those forms from which all other forms can be deduced by the application of certain rules. Take, for example, the English verbs look and write. We know that the -s forms are looks and writes, as only very irregular verbs have irregular -s forms (is and has). Similarly we know that the -ing forms are looking and writing (the latter with the mandatory removal of the final -e). But for the past tense forms, we need to know if we are dealing with regular verbs (looked and *writed[2]) or otherwise. The reality, of course, is that looked is indeed the regular past of look, while write is an irregular verb which does not have *writed but instead wrote as its past form. Furthermore, we know that looked also functions as the past participle as look is a regular verb, but that we need to know all three principal parts of write in order to get its past participle, written. So now we have our entire irregular verb summarised as: write, wrote, written.
Principal parts vary from language to language. In English they are the infinitive/present tense, past tense and past participle; in Latin they are the present, infinitive, past, and supine, as exemplified in fero, ferre, tuli, latum, to carry, a verb that is clearly composed of the remnants of several others; compare go, went, gone in English, where went was once part of the verb wend.