"nemo aliquid facit nisi qui" + indicative or subjunctive

Question

In another question, a reference was given to Varro:

De subus nemini ignotum, nisi qui apros non putat sues vocari.

which was translated as:

As to swine, everybody knows — except those who think that wild boars ought not to be called swine.

At first, it simply struck me as an odd translation. Reading the Latin I got the impression that Varro is referring to a specific person even mocking him a little. Influenced by a recent reading about est/sunt qui constructions, in which the verb inside the clause is usually in the subjunctive case, thus producing a relative clause of characteristic (some examples in A&G 535.1); however the construction sunt/est qui can also come with the indicative. A good take on that can be found in a note on Horace Odes 1:3 (sunt quos curriculo pulverem Olympicum ...):

Sunt quos] The Greeks say ἔστιν οὕς. The indicative is used with ‘sunt,’ or ‘est qui,’ when particular persons are alluded to, as here the Greeks in opposition to the Romans. So Epp. ii. 2. 182: “Argentum — sunt qui non habeant, est qui non curat habere,” where, by the latter, is distinctly indicated the wise man. Here Horace alludes to the Greeks of former days, and is led to refer to them, because this was the chief subject of Pindar’s poetry.

I expected a specific person Varro is referring to because he used the indicative inside the clause. However, this is clearly wrong since it seems that usually nemo aliquid facit nisi qui comes with the indicative (while the simple nemo est qui usually comes with the subjunctive).

That leads me to questions:

1. What is the difference between the subjunctive and indicative inside the nemo alquid facit nisi qui clause. Is it the case it must agree with the mood of the verb of nemo?

2. Is it possible to use this construction to refer to specific/particular persons that are exception to the nemo?

Answer 1

About your question 2, yes, in nemo... nisi constructions, nisi indicates a restriction to the exception. The reference of the qui... clause (see below) can receive either a generic or a specific interpretation.

Now for question 1.

est/sunt qui + subj. is a special construction which means :

"there exist(s) someone/some people such that...": the subjunctive mood here conveys the notion of consequence.

In qui apros non putat sues vocari and nemo... nisi qui... the relative pronoun introduces what is called a free or substantive relative, which is a stand-alone referential relative clause.

If no circumstantial nuance is involved, those clauses must be in the indicative.

If the main verb is itself in the subjunctive, then the verb of the free relative must also be in the subjunctive, because you got a modal context; so yes, it "agrees" :

Si non veniam, nemo sciat quare, nisi qui me bene cognoverint.
"If I did not come, no one would know why, except those who knew me well."

Note that substantive relative clauses are not to be confused with interrogative clauses, which must generally be in the subjunctive.

See the following contrast :

Non audivit qui ad convivium veniebant.
"He did not hear those who were coming to the feast."

Non audivit qui ad convivium venirent.
"He did not hear of which people were coming to the feast."

Answer 2

THIS ANSWER IS WRONG, but might be useful as it contains some important discussions in the comments thus is still open

Thanks to @Vincent Krebs answer and investigating many example, I would like to lay the answer thus:

My primary issue was misjudging the qui in "nemo ... nisi qui". this qui is not a regular relative like vicinus, qui ad me heri venit, magnus est. rather it points back, so I suspect, to the nemo to be the subject of the new condition clause started by nisi. In other words this qui does not describe or define nemo. it is nemo itself.

Indeed there are several examples that this qui is removed:

Contumelia a contemptu dicta est, quia nemo nisi quem contempsit tali iniuria notat; (The word “contumely” is derived from the word “contempt,” for no one outrages another by so grave a wrong unless he has contempt for him; Leob)

or from Cicero

facere nemo poterit nisi eruditus

In one example we can see that qui is like is or ea (i.e., pointing back to a predefined subject):

Quod facienda quoque nemo rite obibit nisi is, cui ratio erit tradita (Because no man can duly perform right actions except one who has been entrusted with reason; Leob)

Moreover, it is important to note the restrictions to the nemo is by condition not by description nor exception, this is why translation like "everybody knows — except those who", while might sometimes work seamlessly as in our example, is somewhat misleading if one to trace back the Latin. Indeed, in some cases this formula simply does not work so well. Consider this example from Seneca:

nemo uxorem duxit, nisi qui abduxit.

It can't really be translated as "no one marries a wife except those who take here (from another). No two groups are implied in the Latin. It would be more accurately translated as "no one marries unless he takes .."

So back to the questions:

2. Is it possible to use this construction to refer to specific/particular persons that are exception to the nemo?

It seems impossible. nemo is already particular/definite grammatically. the subject inside the clause from what I've see must be (and it makes sense given the discussion above) - just as nemo - singular. We cannot refer to specific persons in this construction. A possible way to go would be use praeter instead of nisi like:

si aperte, cur non omnes ferrum habuimus? cur nemo praeter eos qui tuum speculatorem pulsaverunt? ( If it was openly, why did we not all have weapons? Why did no one have them except those who manhandled your spy?; Leob).

1. What is the difference between the subjective and indicative inside the "nemo alquid facit nisi qui" clause. Is it the case it must agree with the mood of the verb of nemo.

I guess this follows the regular conditionals rules. effectively, in the classical examples I saw there is agreement in the verb mood between the nemo's inside and outside the clause.