In chapter X: Canis amicus hominis est, ea bestia fera non est.
Does gender of the demonstratives switch like this in Latin? I was a bit surprised that ea would agree with bestia fera rather than canis amicus.
You have two independent clauses without a conjunction (asyndeton, if you like), each with its own subject and verb. While bestia could be seen in apposition to canis, it’s still feminine, and any adjective has to agree with it in number, gender and case. Ea modifies bestia and not canis.
Oh, thank you. He means “this beast is not wild”. I had thought “it is not a wild beast.”
Ὁ κύων φίλος τοῦ ἀνθρώπου ἐστίν, ἐκεῖνο τὸ θηρίον ἄγριον οὐκ ἔστιν. 
In XII, I’m seeing a lot if ac, which is glossed “= atque”. I assume it’s parallel to “nec = neque”. Given the unpronounceable-looking nature of ac, are these abbreviations rather than synonyms?
Wiktionary call it apocope (ἀποκοπή), so may be it is a variative form of the word?
I don’t know what you mean by “the unpronounceable-looking nature of ac”. But if I understand your question correctly, ac is not Ørberg’s abbreviation of atque, they are alternative forms of the conjunction, both used widely. From the online Latin dictionary Glossa: atque or āc (atque is used before vowels and consonants, ac, in class. lang., only before consonants).
Yes, equivalent to nec and neque. Again from Glossa: nĕ-que or nec (used indifferently before vowels and consonants. The notion that nec in class. prose stands only before consonants is wholly unfounded. …).
Well, it looked unpronounceable last night well I was up late with an upset baby and was thinking about it. Now it doesn’t seem so bad. Is it ac or āc? Ørberg gives the first, and Lease claims the same in “The Use of Atque and Ac in Silver Latin” (1902).
Is it ac or āc?
I definitely do not know. It was probably āc when your baby was agitating. But here’s a challenge I’ll throw out there (without knowing the answer): Is there a hypothetical use and position of ac in poetry that would tell us whether its ă or ā?
Hopefully Ørberg covers the Latin rules for scanning poetry before too long. I can see that it’s similar to Greek, but not exactly the same, and I’d like to know all of the exceptions that I should keep my eyes out for.
Two great resources that I’ve found useful so far: 1) EXERCITIA LATINA all online with some basic Javascript to correct your answers 2) FABELLAE LATINAE with more reading practice
OLD does not mark it as long:
However, as Randy implied, if it’s always before a word beginning with a consonant, in poetry it will practically always be scanned as long by position, so poetry is of no help here. However, the combination -cr- does not always lengthen the preceding vowel, so if somewhere we could find ac followed by a word beginning with -r-, that might answer the question. I could find no such combination in a quick search of the Aeneid or the Metamorphoses.
In chapter XVIII: facere/fierī. And apparently, despite being an -ere verb, it conjugates to faciunt? Not to mention fit/fīunt. Is this simply irregular, or is there rhyme and reason to it?
Verbs of the Third Conjugation in -iō Facio is one of them
For the passive see: fīō , fiĕrī, factus sum,
If you want everything in Latin, you should get
Elementa linguae et grammatica latinae
Third conjugation -io’s are a subcategory of the third conjugation, perfectly regular except for having an -i- in the stem (but they really help with the fourth conjugation, which is practically the same with a few small difference, such as the -i- being long when it can, i.e., capis (you take) vs. audīs, (you hear), and of course the infinitive. As for fiō, for whatever reason it serves as the passive of of the present system of faciō, one of the few irregularities you will encounter in Latin.
I agree that Cornelia is a good companion to Lingua Latina. Cornelia was part of a 3 book curriculum for beginners in Latin, each of 40 chapters which synchronize well. Cornelia works best when read in parallel with its companion books, The New Latin Primer and Carolus et Maria. I’d estimate that the series teaches 500 or so words and gives the grammer to the level of the first 10 or 15 chapters of Lingua Latina. It’s a good confidence builder.
New Latin Primer: https://archive.org/details/ANewLatinPrimer
Cornelia: https://archive.org/details/MN40039ucmf_6
Carolus et Maria: https://files.vivariumnovum.it/edizioni/libri/dominio-pubblico/Fay%20-%20Carolus%20et%20Maria.pdf
And audio book of Cornelia.
There are others in this series as well. For example, “Acta Muciorum,” and, “Julia.”
See here: https://www.amazon.com/Acta-Muciorum-Latin-Mima-Maxey/dp/1946943010/ref=sr_1_fkmrnull_2?keywords=mima+maxey&qid=1556369834&s=gateway&sr=8-2-fkmrnull
Hope this helps,
Cathexis
In chapter XX.3 of Fabellae Latinae, “absentem cantat amīcam” was surprising to me. I had only seen the verb used transitively before this. Is “absentis amica” a specific song?
Yes, the transitive use (which this is) is very common in poetry. absentem cantat amicam is the second half of a hexameter. Google tells me it’s a modification of a line from Horace’s Satires, the one about his trip to Brindisi, with the nocturnal emission, I think.