Chosing between the different ways to make an adverb

Question

I was thinking of translating “That is not dead which can eternal lie” into Latin for fun, thinking it was not only applicable to Cthulhu, but to the Latin language itself :)

But I got stumped trying to pick the adverb. As described in Could an adjective be used like an adverb in Latin?, there are 3 ways to turn an adjective into an adverb in Latin, but I am unsure if all 3 ways are applicable to all adjectives, nor whether they mean the same thing.

Id nōn est mortuum quod aeternō iacere potest.

Id nōn est mortuum quod aeternē iacere potest.

Id nōn est mortuum quod aeternum iacere potest.

Are all three valid? If not, why not? If yes, what's the difference?

Answer 1

It looks like aeternē was rarely used in Classical Latin. The PHI corpus shows only three matches, two of which are in grammatical discussions explaining that aeternum means aeternē.

The third is in Cicero De Natura Deorum 1.109.6:

quo modo enim probas continenter imagines ferri, aut si continenter, quo modo aeterne?

The Lewis and Short entry for aeternus has an entry for adverbial expressions that mentions aeternum and aeternō, and also the phrase in aeternum:

Adv. phrases.

1. in aeter-num.
   A Lit., forever, everlastingly: et vivat in aeternum, Vulg. Gen. 3, 22: hoc nomen mihi est in aeternum, ib. Exod. 3, 15: Dominus in aeternum permanet, ib. Psa. 9, 8: vivet in aeternum, ib. Joan. 6, 52: Tu es sacerdos in aeternum, ib. Heb. 5, 6: non habebit remissionem in aeternum, ib. Marc. 3, 29.—
   B Meton., of indef. long time, forever, always: urbs in aeternum condita, Liv. 4, 4: leges in aeternum latae, id. 34, 6: (proverbia) durant in aeternum, Quint. 5, 11, 41: delatores non in praesens tantum, sed in aeternum repressisti, Plin. Pan. 35: (famulos) possidebitis in aeternum, Vulg. Lev. 25, 46: (servus) serviet tibi usque in aeternum, ib. Deut. 15, 17: ut sceleris memoria maneat in aeternum, Lact. 1, 11.—
2. aeternum.
   A Lit., forever: sedet aeternumque sedebit Infelix Theseus, Verg. A. 6, 617: ut aeternum illum reciperes, Vulg. Phil. 15 (prob. here an adv.).—
   B Meton., of indef. long time, forever, always: serviet aeternum, Hor. Ep. 1, 10, 41.—
   C Of what is continually repeated, constantly, again and again (as in colloq. Engl., everlastingly, eternally): glaebaque versis Aeternum frangenda bidentibus, Verg. G. 2, 400: ingens janitor Aeternum latrans (of Cerberus), id. A. 6, 401.—
3. aeternō, meton., of indef. long time, forever, perpetually: viret aeterno hunc fontem igneum contegens fraxinus, Plin. 2, 107, 111, § 240: BVSTA TVTA AETERNO MANEANT, Inscr. Orell. 4517.

I don't have enough reading experience in Latin to have a sense for the differences, but a more experienced Latinist, Kingshorsey, made the following observation in a Reddit post:

The most common adverbial form is in aeternum, followed by aeternum and aeterno.

(2019/1/23 reply to "Translation request: Licet amētis permansimus, nunc, et aeternum.")