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According to Wiktionary, prodigo is a verb which etymology comes from "pro + ago". The same is suggested by L&S. However, I cannot see how ago fits here. The conjugation of this verb seems at odds with that of prodigo. Is there some mutation involved? How did proago become prodigo?

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Before vowels the prefix pro- becomes prod-. In addition to prodigere, we have prodire, prodesse, and maybe others that I forget now. Where the -d- comes from is another question, but for practical purposes of learning Latin one can just learn the prevocalic version of the prefix.

It is very common that a short a in a short first syllable of a verb becomes i when a prefix is added. For example, consider ex-facere > efficere. This has happened with agere, too, turning it into -igere after (one-syllable) prefixes.

Once you change the short a into a short i in the present stem (otherwise the vowel or syllable is long) and remember to put the extra d in pro-, the conjugations of agere and prodigere match perfectly.

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Joonas's answer is entirely correct, but just to add onto it a bit:

The way I learned it, prōd is an archaic form of prō. You'll also sometimes see pōr as in pōrrigō, pōlluō, etc: these three all came from different forms of the same PIE root.

In the end, prō was the one that won out, and so that's the only one you'll see used as a preposition. But all three of them show up in verb prefixes. Prōd was generally used in front of vowels, because Latin doesn't like vowels in hiatus (next to each other with nothing in between). And pōr was generally used before l or r, where prō would be awkward (forms like *prōrigō are somewhat unwieldy).

(It's also possible that prōd turned into prō, because Latin generally got rid of final ds: that's how we got filiā < filiād and < med (mēd?). But that's just conjecture; I don't actually know where the different forms came from.)

Re the vowel, Old Latin used to stress the first syllable of a word. And during this time, short vowels in non-final unstressed syllables (that is, any syllable besides the first and the last) got "reduced". The following rules aren't watertight, but they're decent guidelines:

  • If the vowel was next to an r, it became e
  • If the vowel came before two consonants, it became e.
  • If the vowel was next to a "dark l" (short l not touching a front vowel), it became u
  • Otherwise, it became i

You see this in a lot of prefixed verbs, like faciō ~ afficiō ~ affectus, or ce ~ occultus. The a was the original vowel here, but it only survives when it's in the first syllable of the word. (You also see this reduction a lot in reduplicated perfects, like ca ~ ceci. Basically any time you have a prefix before a short vowel.)

And that's what happened here: the original vowel was an a, but when it was pushed out of the first syllable, it got reduced to an i.

Once you make that change, you'll see that the conjugation actually lines up quite nicely! It's just one of the quirks of Latin that you have to get used to.

  • Great answer! One point, though: It seems to make a difference for the vowel change whether the prefix has one or more syllables. We have circumagere, but circumiacere also has the reduced variant. Maybe the new stress on the first syllable of the prefix is far enough so that a secondary stress can be kept on the first syllable of agere in old Latin? – Joonas Ilmavirta Apr 1 at 15:36
  • @JoonasIlmavirta I think those forms are later derivations, since the second syllable of circum doesn't get reduced either. Alternately, L&S say that verbs prefixed with circum are often written as two separate words, so the Romans might not have thought of them as truly joined. – Draconis Apr 1 at 16:06
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    @ColinFine Oh, sorry—what I meant wasn't that the writing made the sounds not change, but that the Romans wrote them separately because they thought of circum and as two separate words. In which case the sound changes, which happened only in words of 3+ syllables, would have left them alone. – Draconis Apr 1 at 23:34
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    (In other words, the writing reflects the Romans' pronouncing them as separate words, not the other way around.) – Draconis Apr 1 at 23:35
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    I've always seen prod- written as prōd-, with a long vowel. The first syllable of prodigo scans as long, and I don't think that it's a case of the d acting as a coda consonant that makes a heavy syllable with a short vowel, because the similar example of redigo has a light first syllable. – sumelic Apr 2 at 3:34

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