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Why do Latin to English dictionaries list three forms of a Latin verb? I've seen this other places like grammar books too. For example:
sedeō, sēdī, sessum: to sit.
There's no Latin keyboard for my phone, so please excuse the mixed up symbol for the long e in the second word.
Actually, Latin dictionaries tend to list four forms of a Latin verb. These forms are known as "principal parts." So the "official" listing for your example consists of four principal parts:
sedeō, sedēre, sēdī, sessum [or "sessus," depending on which tradition you follow].
The first word means "I sit," the second one "to sit," the third "I sat," and the fourth—well, the fourth is a little complicated in the case of this verb, so let's leave it alone for now, but it's used in a few different constructions.
The easiest way to explain why dictionaries list these four parts is to look at English verbs. Most English verbs are regular: if the present tense is "walk," then the past tense is "walked" and the past perfect is "had walked." Same with "rain, rained, had rained." Easy, right?
Well, not quite. If this were the only rule you knew about English verbs, what would happen if you saw the verb "eat"? You'd think that the past tense was "eated" and the past perfect was "had eated." Or you'd come up with "run, runned, had runned."
Now, in English this usually happens just with irregular verbs, but in Latin it operates with most verbs. So for any given verb, there's no way to know what the past tenses are except to learn them, just like you have to learn "eat, ate, eaten."
Here are some other verbs the present tense of which seems to look like sedeō:
habeō, habēre, habuī, habitus
ineō, inīre, inīvī, initus
gaudeō, gaudēre, gavīsus sum, —
videō, vidēre, vīdī, vīsus
So the third word in the dictionary entry lets you know whether the past tense of the verb sedeō is seduī, sedīvī, sevīsus sum, or sēdī.
There are rules of thumb—for example, there are a lot of verbs that look like habeō, habēre, habuī, habitus, so if you don't know, it's not a bad guess. And verbs that start out amō, amāre almost always end amāvī, amātus. But there are exceptions! So the dictionaries just play it safe and give you all four parts for every verb.
If you want to be able to conjugate a new verb, one form is not enough. If you have those three and the conjugation number (some dictionaries give the present infinitive instead of the number), you can figure out any form of the verb.
The verb you mention has three stems (or principal parts): sed-, sēd- and sess-, as one can read from those three forms. All other forms are formed by attaching endings to these stems. Many Latin verbs are irregular in the sense that it is next to impossible to guess the correct stems with great certainty. The endings themselves are rarely irregular. Therefore, apart from the most irregular verbs (like esse), those stems are sufficient and necessary for finding all the forms.
Essentially the same thing happens with nouns. One form is not enough, but if you are given singular nominative and genitive and the gender of the word, you can find all other forms.
