A translation labeled “Neo-Elvish” just means that Tolkien didn’t do the translation, someone else did. That means that ANYTHING not directly attested by Tolkien is Neo-Elvish.
Let’s take the Sindarin word “linnon – I sing.” Now, through analysis of other forms of the verb like “linnathon – I will sing” and through analysis of other verbs like “nallon – I cry out” we can deduce that “linnon” is made of two morphemes, “linna– to sing” and “-on – I”. These two separate morphemes are Neo-Elvish because they aren’t attested on their own. When we have a verb like “minna- to enter” we can put it together with “-on” to make “minnon – I enter.” Being able to break down and analyze the grammar means that we can generate more phrases that follow the attested examples.
What Neo-Elvish is NOT
Randomly making up our own grammar and vocabulary and calling it Elvish. Everything in Neo-Elvish must be derived from other Elvish languages that Tolkien made, or it is rejected by the community at large. This isn’t something that we take lightly, and we argue over fan-made words extensively.
Fan-made vocabulary is marked with an asterisk to differentiate between it and a word made by Tolkien. We make new vocabulary in several ways
Making new words the way that Tolkien did. This means taking the ancient roots that Tolkien listed (there are hundreds of these), making an ancient compound word, then putting the compound through the phonetic history of the target language.
For example, we’ll make a Neo-Quenya word for “to blush” by modeling it after a similar word, “niquita– to whiten.” We take the ancient word “karanĭ – red” and add the causative suffix “-tā“, then put the new word through Quenya sound changes to make “*carnita- to redden, blush, make red.”
Just putting two words together to make a new one. For example, we made a Neo-Sindarin word for “Hatter” by echoing the form of “círdan – ship wright” putting together the words for “carab – hat” and “tân – maker, smith” to make “*carabdan – hatter.”
Both of these methods require extensive knowledge of Tolkien’s languages, their histories, and how Tolkien himself made words. We’re doing our best to imitate Tolkien’s process so we can make vocabulary that fits into his languages, that he could possibly have made himself.
Of course, this means that there are some things that we can’t translate. We accept these limitations because we want to be authentic and true to Tolkien’s versions of the languages as we can.