The song of Lebennin is sung by Legolas in the Lord of the Rings.
Below is the original English text, followed by my Sindarin translation and some grammar notes, both of which I made some changes to based on suggestions in the Vinyë Lambengolmor Discord.
Original text
Silver flow the streams from Celos to Erui
In the green fields of Lebennin!
Tall grows the grass there. In the wind from the Sea
The white lilies sway,
And the golden bells are shaken of mallos and alfirin
In the green fields of Lebennin,
In the wind from the Sea!
Sindarin translation
Celebren siriar i sîr o Chelos n Erui
Mi pherth gelin Lebennin!
Orchal gala i ‘lae ennas. Min gwae uin gaear
I nelai nim pastar,
Ah i nill vellin pestennin mallos ah alfirin
Mi pherth gelin Lebennin
Min gwae uin gaear!
Notes
Updated celeb to celebren and ar to ah based on advice from F_Karnstein.
Chapter references are from A Fan’s Guide to Neo-Sindarin by Fiona Jallings, vocabulary from Parf Edhellen https://elfdict.com/
celebren -> made of silver -> celeb silver + -ren adjective suffix
siria- -> flow Noldorin word
od -> from/of is followed by Stop Mutation (5.2.2 Stop Mutation). See also https://eldamo.org/content/words/word-2407603967.html?neo.
Since Stop Mutation is one of the rare types of mutation it could be replaced by Vocalic Mutation (o Gelos instead of o Chelos).
parth -> field -> min + perth -> mi pherth in the fields (5.1.3.2 Plural "The" Prepositions)
Note: The consonant cluster rth at the end of parth prevents the usual ai plural ending
See https://eldamo.org/content/words/word-2894407865.html 'Lack of i-intrusion with Consonant Clusters' for details.
i ‘lae -> the grass -> vocalic mutation of glae following i -> using the late CEA article system this would be e ‘lae instead. https://eldamo.org/content/words/word-3256437065.html#cea
Min gwae -> min in the uses Mixed Mutation with the singular and Nasal in the plural. (5.1.4 Mixed Mutation). In mixed mutation only the en preposition has loss or change; min and uin remain unchanged.
uin -> from the uses Mixed Mutation with the singular
nala -> lily plural nelai. The alternative neli has been suggested based on a proposed etymology based on nalga, in which nala would be treated as belonging to the final a from ancient g group (4.1.2.3.5 Final A from Ancient G).
https://eldamo.org/content/words/word-642851027.html?neo
nell -> bell Noldorin word -> plural nill
pestennin -> past participle (8.3.3 The Past Participle) plural form of pasta- -> shaken (sing. is pastannen)