The fourth line of Ae Adar Nín, Tolkien’s Sindarin translation of the Lord’s Prayer (VT44/21). The first word caro is the imperative form of the verb car- “to do”. The third word is the definite article i “the”, followed by innas “will” and the possessive pronoun lin “your”, with the adjectival element following the noun as is usual in Sindarin.
The function of the word den in this phrase is unclear. Bill Welden suggested (VT44/25) that is a marker of the passive voice (“thy will be done” instead of “do thy will”), and is the lenited form of ten, either a 3rd-plural pronoun “they” (as in impersonal English phrases such as “they say ...”) or a neutral pronoun “it”. Since “they” appears as di (← ti) elsewhere in the prayer, I think the second possibility is more likely.
See the entry for the second line of this prayer for a discussion of the use of the definite article i before the possessed noun in this phrase.
Decomposition: Broken into its constituent elements, this phrase would be:
> car-o den i innas lin = “✱do-(imperative) it the will yours”
A word for “pool” or “lake”, appearing as an element in the name Linaewen “Lake of Birds” (S/119; UT/401). As an independent word it is probably ✱lîn, and it is probably a derivative of the root √LIN of similar meaning (PE17/145, 160).
Conceptual Development: The earliest obvious precursor to this word is ᴱN. lhuin “pool” in Early Noldorin Word-lists of the 1920s (PE13/149). In The Etymologies of the 1930s it appeared as N. lhîn “pool”, a derivative of the root ᴹ√LIN “pool” and cognate to ᴹQ. linya (Ety/LIN¹). Remnants of the ancient y can be seen in its (lenited) class plural liniath in the name N. Hithliniath “Pools of Mist” (Ety/KHIS; LR/262). It cannot be a direct cognate of its Quenya form, however, at it has no signs of a-affection. Christopher Tolkien mentioned the name element lin “pool, mere” in The Silmarillion appendix (SA/lin), though whether it was intended to be a primitive form or a Sindarin word isn’t clear.
Neo-Sindarin: I think it is best to simply adapt the Noldorin form into (Neo) Sindarin as lîn, perhaps derived from primitive ✱linyē and hence with class plural liniath.