okay, "noctu" is the indeclinable ablative case. but in the first question, duo writes this indeclinable ablative case instead of the nominative under the picture. the nominative is "nox" and its formal ablative is "nocte." it is a syncopated i-stem.
It's not ablative, because it's not a noun or adjective. Noctu is an adverb. I'm not sure what you mean by "formal ablative". A word either is ablative or isn't, no formality.
nope. have you ever read plautus or naevius? you can clearly see the constructions as "hac noctu, de noctu," etc. in them. noctu was used as an ablative first and then became an adverb. so, noctu is an archaic form of the ablative and nocte is the formal.
According to P.W.Glare in the Oxford Latin Dictionary, it exists both as ablative and adverb. Naturally, there are other ablatives that can be used as adverbs, e.g. "falso", "primo" and "vulgo".
I stand corrected on it being ablative, but that still doesn't make one formal.
ok. i see that i couldn't explain my point clearly. maybe "formal" is not the right word to describe it (i have some problems with my english vocabulary). what i try to mean by the formal is that we see its "nocte" form on almost every classical text. and we know that "nox" is a syncopated i-stem word, so the ablative should be "nocte" according to its linguistic properties. but "noctu" was somewise invented by Naevius for certain poetical reasons, at least we know it that way from the survived texts. we can also see that noctu form on the poems of Vergilius, Ovidius etc. that is to say that noctu is an archaic (Naev.+) and poetical form. and i thought that the form which is not poetical could be named as formal.
I would avoid using the term formal. Users are accustomed to hearing formal and informal for tú vs usted or similar pronouns in the Romance languages that many of them have studied.
I would call nocte common.