What is the optative?

Question

Some conjunctive forms end in -im (and -is, -it, -imus, -itis, -int), but this is rare. The examples I recall are sim, possim, velim, nolim, malim, and duim (alternative to dem). These forms are sometimes explained to derive from an optative mood which Latin has lost and whose functions were absorbed into the conjunctive. The grammars I have seen do not discuss the optative, perhaps because it seems not to have survived in classical Latin. By this I mean that no classical Latin word seems to contrast optative and conjunctive morphologically.

I would like know more about the optative.

• Did all verbs have optative forms? Were the endings always the ones I listed above?
• Did optative and conjunctive coexist and have distinct roles and forms?
• Do we know how the optative endings were attached to regular verbs like amare, videre, trahere, capere, and audire?
• How was the optative used? I assume that it was used to express positive and negative wishes, but I am not sure.

In short, the question is: What was the old optative like in Latin (if there was one)? (If there never was an optative in Latin and it has to be traced back to PIE, then — but only then — I am interested in the PIE optative.) Answers don't need to be overly detailed; I can ask for further details in follow-up questions. I am looking for a big picture here.

Wikipedia gives a short description of the Latin optative with no citations:

Likewise in Latin, the newer subjunctive is based on the Indo-European optative. With this change in Latin, several old subjunctive forms became future forms. Accordingly, the prohibitive (negative desire and prohibition) was formed with the combination of ne + verb form in the optative present.

Note that I mean optative forms, not the optative function of the conjunctive (coniunctivus optativus).

Answer 1

Latin as we know it never had an optative mood as distinct from the subjunctive, so this answer will be largely about Proto-Indo-European (PIE), which did.

PIE had both a subjunctive mood and an optative mood, with different forms and functions. The optative was formed with the suffix *-yeh₁- ~ *-ih₁-. (The sound written as *h₁ was probably either a glottal stop or [h]. The reason there are two forms is the PIE ablaut system, in which most morphemes had more than one variant showing different vowel grades.) Judging from languages that preserve a distinct optative, such as Greek and Vedic Sanskrit, the meaning of the optative seems to have had to do with potentiality and wishes: in these languages, the optative is used in meanings like "may it happen!" and "it would (potentially) happen" (often used in conditions, counterfactuals, and similar constructions).

Formally, the optative is thought to underlie, partly or wholly, the Latin present subjunctive. The forms in -im which you cite (sim etc., to which can be added faxim, dixim) come from PIE *-ih₁-. Also, in the first conjugation, the subjunctive suffix -ē- may come from PIE *-yeh₁-. The -ā- of the other conjugations is a longstanding unsolved problem, but some have thought it too ultimately comes from the PIE optative suffix. This may be why the Latin subjunctive has, among others, the functions of the PIE optative (viz. optative subjunctive, potential subjunctive, subjunctive in conditions).

As for the PIE subjunctive, it was formed with the suffix *-e/o- and is thought to be continued in the -ē- that marks the future of the Latin 3rd-4th conjugations.

At what stage between PIE and Latin was the optative/subjunctive distinction lost? Certainly before the oldest Latin, since such a distinction is not attested in any Latin text or inscription. The other Italic languages -- or at least Oscan and Umbrian, the ones about which we know the most -- also do not, to my knowledge, preserve the distinction. This would suggest that the loss may predate Proto-Italic. However, @AlexB in a comment quotes Rix as thinking that Proto-Italic still had a distinct subjunctive and optative; I assume Rix had good reasons to think so, but I don't know what they are.