Principal parts (verb)

The principal parts of an irregular verb 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) or otherwise. The reality, of course, is that looked is indeed the regular past of look, but that write is an irregular verb which does not have writed but wrote as its past form. Furthermore, we know that looked also functions as the past participle in the regular verb, but that we need to know the principal parts of write in order to get its past participle, written. So now we have our entire irregular verb: write, wrote, written.