Commentator is just basically wrong, but just by popular of it greatly eclipses correct usage. It's one that even the dictionary gives up on, but just using what you were taught in 3rd. grade- A dictator dictates, a commentor comments, nobody commentates.
Therein lies your problem: in 3rd grade they don't tell you the whole story (and rightly so).
Your explanation only applies to "commentat
er", which can't be a word because, indeed, "to commentate" isn't either: the English native "-er" agent suffix is added to the base form of the word ("loan" -> "loaner") with the opportune spelling adjustment in written form ("love" -> "lover", "ship" -> "shipper").
However, "commentat
or", with an "o", is an originally Latin word, sporting Latin agent suffix "-or", which worked (
in Latin) by being added to the supine (a sort of verbal noun) form of a verb:
commentari (="to comment"), sup.
commentatum (roughly ="the commenting") ->
commentator (="one who comments"). The word was borrowed into English wholesale, and its existence in your language doesn't depend on "to commentate" (not) being there.
There are, in fairness, complicating factors:
1) while some English verbs ultimately derive from Latin verbs in present/infinitive form (Lt.
convenio,
convenire -> Eng. "to convene"), others were borrowed from the perfect participle/supine form (Lt.
dictare, p.p.
dictatus/
-a/
-um, sup.
dictatum -> Eng. "to dictate")
2) some Latin verbs were intensive forms of other verbs, built upon the supine of the latter:
commentari itself was one (of
comminisci, sup.
commentum), as was
dictare (of
dicere, sup.
dictum);
the above explains the apparent repetition of (pastness, or patiency -inspiring) -t-'s, but also the fact that "dictator" superficially appears to be the agent noun of "dictate", and to have been formed by simply appending "-or" to it: if
dictare had been borrowed as "to dict" in English, which it happens to have not, that equation wouldn't work now.
Finally,
3) the Latin suffix "-or" is sometimes applied
in English, in lieu of native "-er", to verbs that are recognised as Latinate, directly to their base form (i.e. just as Germanic "-er" works): "survivor", "resistor", "licensor". This inconsistent practice – possibly, in part, influenced by French suffix "-eur", itself derived from Latin
-at-or but with no trace left of -t- (typical of the supine form of many verbs as we've seen) due to historical phonetic erosion – disrupts the above-described pattern where English native, still productive "-er" and Latin fossilised "-or" attach to verbs differently;
4) you guys pronounce those two bastids the same!
(Usually, that is: I'm aware of "sensor" being pronounced "sense or" by some, notably on
Star Trek... "Bailor" too.)