In chapter XXI, lines 115-116, of Lingua latina per se illustrata. Familia Romana (page 167) there is this sentence:

Nōn difficile est mātrem Mārcī fallere!

Its meaning is clear to me, but I'm not sure about its interpretation from the grammatical point of view.

I believe that difficile is the nominative singular neutral form of the adjective difficilis (that was for me something difficult to understand: at first, I thought it was an adverb). Then, I think (nōn) difficile est an impersonal construction similar to necesse est. There is an accusative: mātrem. So, I thought this was an example of the so called accusativus cum infinitivo structure. But I'm not at all sure about that. If I understand correctly, in the accusativus cum infinitivo construction, the accusative is the subject of the verb in the infinitive form. However, by context, in this case mātrem Mārcī is the direct object of fallere: it's Marcus who has cheated on his mother. Could someone help me clarify my doubt?

  • Related: latin.stackexchange.com/q/21621.
    – Charo
    Commented Oct 28, 2023 at 10:21
  • 1
    @JanusBahsJacquet: Is the copy of the book on archive.org really unauthorised? I will accept your edit just in case it's that way.
    – Charo
    Commented Oct 29, 2023 at 16:07
  • 1
    Yes, it is; I happen to know this because I work with the publisher that published the first version of the series with Hans Ørberg back in the ’80s and now function as distributors in the Nordics. I wrote to the US publisher (whose digital version had been uploaded) yesterday to notify them, and they wrote back to say they’ve sent a cease and desist to Archive.org for it. (Incidentally, your mention didn’t generate a notification for me, though it ought to, since I’d edited the question. Perhaps it only works after the edit has been approved.) Commented Oct 30, 2023 at 21:07

2 Answers 2


This is not ACI. In an ACI the infinitive is often (a part of) the object, whereas here it is the subject. An ACI comes with a dominant structure saying something to the effect of "I know that", but there is no such preface here.

What often helps a lot is to strip a sentence of details to see its core. In this case I'd reduce it to:

Difficile est fallere.
It is hard to cheat.
To cheat is hard.

Add the details and you end up with:

It's not hard to cheat Marcus's mother.

It is not stated in this sentence who the cheater is. Only the cheatee is named, and only in potentiality.

  • Joonas, I downvoted your answer accidentally (I was reading it from my mobile and it slipped off my hands and I clicked on the down arrow by mistake). I've tried to solve my error but this message has shown up: "You last voted on this answer 1 hour ago. Your vote is now locked in unless this answer is edited." Could you then please edit your answer? For example, here is a suggestion for editing: your statement "In an ACI the infinitive is (a part of) the object" is not correct for all the cases: e.g. cf. Charo's ex. necesse est: note that in Necesse est te abire the ACI is the subject.
    – Mitomino
    Commented Oct 28, 2023 at 14:34
  • @Mitomino Thanks! I made a quick edit (ACI is often but not always the object) that should allow you to undo your vote. I am on my phone too, and will try to edit a bit more when I get to my computer.
    – Joonas Ilmavirta
    Commented Oct 28, 2023 at 15:22
  • OK! Vote undone!
    – Mitomino
    Commented Oct 28, 2023 at 17:49

difficile is a neuter singular adjective which agrees with the infinitive fallere being used substantively as a nominative. A classical analog is this sentence from Ovid:

heu! quam difficile est crimen non prodere vultu! ("Alas! How difficult it is not to show guilt in the face!")

Marci is genitive modifying matrem. The mother of Marcus.

(In accusative with infinitive, the word in the accusative is the subject of the infinitive and the word in the infinitive is at the same time the object of some other verb. So that is not applicable here because matrem is the object of fallere, not its subject.)

One way to figure out grammar in these situations is to use a book called "Valpey's Delectus" which is publicly available and has model sentences. Find a model sentence that matches the grammar you are trying to decipher, then look at the notes which will exactly explain the grammar. For example, the notes to the analog in this case read as follows:

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