From other language study, you are familiar with the concept of PRINCIPAL PARTS—that bare-bones set of information that enables us to predict how any given verb will perform. In French, for instance, if you want to control the verb that means “see,” you have to know the forms “voir, voyant, vu, je vois, je vis.” A foreigner learning English can solve the mysteries of our verb by learning the three forms “see, saw, seen.” For many centuries, Latin verbs have been learned by means of FOUR principal parts; in this case, video, videre, vidi, visus. You’re perhaps aware that our word video is the Latin for “I see”—though there may be more people alive today, using that word in English without knowing its source, than all the speakers of Latin in human history. If you have had any kind of a close encounter with the ancient world, you will know that Julius Caesar is supposed to have said, “veni, vidi, vici” (“I came, I saw, I conquered”); therefore vidi must mean “I saw.” But is there any practical reason for us to learn video and vidi? With very few exceptions—video (“I see”), audio (“I hear”), and credo (“I believe”)—1st person Latin verb forms are not reflected in English. Accordingly, of the four traditional principal parts, we need only concern ourselves with two: vidḗre, the PRESENT INFINITIVE (“to see”), and visus, the PERFECT PARTICIPLE (“seen”). These two forms are the source of countless English words. From videre, the present infinitive, come such derivatives as provide and evident; from visus, the perfect participle, come vision, visible, visual, proviso, and the like. Because the one Latin form cannot be predicted from the other, we must learn them both as vocabulary items. That is not a hard task, however, given their clear resemblance to English derivatives; and it is well worth the effort in terms of the increase it will bring to your understanding of English vocabulary.
For the sake of convenience, we shall be using the terms PRESENT INFINITIVE and PERFECT PARTICIPLE, a verbal adjective that is often called the “past participle” by English lexicographers. Although you should be familiar with the labels, there is no reason to know how the two forms were used grammatically in Latin. In fact, all we really need are the present base (or stem) vid(e)– and the perfect base (or stem) vis-. For most verbs, the present base is obtained by dropping the -re of the infinitive, and the perfect base by dropping the -us ending of the perfect participle. Still, it is probably easier and more memorable to learn them as videre / visus than as vid(e)- / vis-.
If you want an almost infallible trick for remembering any Latin perfect participle, THINK OF AN ENGLISH DERIVATIVE IN -ION. By firmly associating vis-us with the English word vis-ion, you can learn it without any painful effort of memory.