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 it 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: 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.