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Why do edo (eat) and edo (generate) have different forms?

So, for example, both are edit in the 3rd person singular. But in the 2nd person perfect, one is edisti and the other is edidisti.

They are both 3rd conjugation, so I guess they are supposed to be following the same paradigm, but there are differences.

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Knowing the four conjugations is useful, but is not sufficient to figure out the full paradigm of a verb. For example, some third-conjugation verbs form their perfects with -s- (dixī), others with reduplication (cucurrī), others with vowel changes (ēgī), and so on. Historically, these go back to a few different constructions in Proto-Indo-European that fell together in Proto-Italo-Celtic (or Proto-Italic and Proto-Celtic if you prefer). So there's no reliable way to predict which one a given verb will use.

So the standard way to learn a Latin verb is to memorize four principal parts:

  • The first singular present active indicative, which is the citation form used to talk about the verb
  • The present active infinitive, which tells you the conjugation, and thus tells you how to conjugate the present system; cut off the ending to get the "present stem"
  • The first singular perfect active indicative, which tells you how to conjugate the perfect system; cut off the ending to get the "perfect stem"
  • The supine or perfect passive participle, which is usually predictable from the perfect stem but not always

The two verbs you're talking about are

  • edō, edere (or ēsse), ēdī, ēsus
  • ēdō, ēdere, ēdidī, ēditus

And as you've seen, you need to know the third principal part to conjugate them in the perfect system; there's no reliable way to predict it from the other parts.


As for why they're like this, historically…

Edō is pretty regular, except for a couple weird present forms (which come from it being an athematic verb in Proto-Indo-European). It forms its perfect stem the same way as agō~ēgī, which is not unusual for the third conjugation.

Ēdō comes from ē- "out" plus dō, dare, dedī, datus "give". In the history of Latin, short vowels in medial syllables (i.e. not the first or last syllable of a word) got reduced, so *ēdare regularly became ēdere, and *ēdatus became ēditus. Synchronically, though, ēdō forms its perfect stem through reduplication, which is also not unusual for the third conjugation (compare currō~cucurrī).

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    Is ēdō from dare "give" (from PIE *deh₃-) or it is from *dʰeh₁-? I find the etymology of Latin -dō, -dere, -didī, -ditum verbs confusing. The reduplication of a non-initial syllable is somewhat unusual (verbs that reduplicate often lack reduplication when prefixed)
    – Asteroides
    Sep 17 at 22:02
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    @Asteroides I thought it was *deh₃ but google is suggesting that, in fact, most prefixed verbs I thought were from *deh₃ are actually from *dheh₁. Fascinating. I'll have to ask a new question about that.
    – Draconis
    Sep 17 at 23:27

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